Light-emitting element, display device, electronic device, and lighting device

ABSTRACT

A light-emitting element containing a fluorescent material and having high emission efficiency is provided. The light-emitting element contains the fluorescent material and a host material. The host material contains a first organic compound and a second organic compound. The first organic compound and the second organic compound can form an exciplex. The proportion of a delayed fluorescence component in light emitted from the exciplex is higher than or equal to 5%, and the delayed fluorescence component contains a delayed fluorescence component whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement, or a display device, an electronic device, and a lightingdevice each including the light-emitting element.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method of driving any of them,and a method of manufacturing any of them.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting substance (an EL layer) is interposed between a pair ofelectrodes. By application of a voltage between the electrodes of thiselement, light emission from the light-emitting substance can beobtained.

Since the above light-emitting element is a self-luminous type, adisplay device using this light-emitting element has advantages such ashigh visibility, no necessity of a backlight, and low power consumption.Furthermore, such a light-emitting element also has advantages in thatthe element can be manufactured to be thin and lightweight, and has highresponse speed.

In a light-emitting element whose EL layer contains an organic compoundas a light-emitting substance and is provided between a pair ofelectrodes (e.g., an organic EL element), application of a voltagebetween the pair of electrodes causes injection of electrons from acathode and holes from an anode into the EL layer having alight-emitting property and thus a current flows. By recombination ofthe injected electrons and holes, the light-emitting organic compound isbrought into an excited state to provide light emission.

Note that an excited state formed by an organic compound can be asinglet excited state (S*) or a triplet excited state (T*). Lightemission from the singlet-excited state is referred to as fluorescence,and light emission from the triplet excited state is referred to asphosphorescence. The formation ratio of S* to T* in the light-emittingelement is 1:3. In other words, a light-emitting element containing acompound emitting phosphorescence (phosphorescent compound) has higheremission efficiency than a light-emitting element containing a compoundemitting fluorescence (fluorescent compound). Therefore, light-emittingelements containing phosphorescent compounds capable of converting atriplet excited state into light emission has been actively developed inrecent years.

Among light-emitting elements containing phosphorescent compounds, alight-emitting element that emits blue light in particular has yet beenput into practical use because it is difficult to develop a stablecompound having a high triplet excited energy level. For this reason,the development of a light-emitting element containing a more stablefluorescent compound has been conducted and a technique for increasingthe emission efficiency of a light-emitting element containing afluorescent compound (fluorescent element) has been searched.

As one of materials capable of partly converting the triplet excitedstate into light emission, a thermally activated delayed fluorescent(TADF) emitter has been known. In a thermally activated delayedfluorescent emitter, a singlet excited state is generated from a tripletexcited state by reverse intersystem crossing, and the singlet excitedstate is converted into light emission.

In order to increase emission efficiency of a light-emitting elementusing a thermally activated delayed fluorescent emitter, not onlyefficient generation of a singlet excited state from a triplet excitedstate but also efficient emission from a singlet excited state, that is,high fluorescence quantum yield are important in a thermally activateddelayed fluorescent emitter. It is, however, difficult to design alight-emitting material that meets these two.

Patent Document 1 discloses a method: in a light-emitting elementcontaining a thermally activated delayed fluorescent emitter and afluorescent compound, singlet excitation energy of the thermallyactivated delayed fluorescent emitter is transferred to the fluorescentcompound and light emission is obtained from the fluorescent compound.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2014-45179

SUMMARY OF THE INVENTION

In order to increase emission efficiency of a light-emitting elementcontaining a thermally activated delayed fluorescent emitter and afluorescent compound, efficient generation of a singlet excited statefrom a triplet excited state is preferable. In addition, efficientenergy transfer from a singlet excited state of the thermally activateddelayed fluorescent emitter to a singlet excited state of thefluorescent compound is preferable. Moreover, energy transfer from atriplet excited state of the thermally activated delayed fluorescentemitter to a triplet excited state of the fluorescent compound ispreferably inhibited.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element that contains afluorescent compound and has high emission efficiency. Another object ofone embodiment of the present invention is to provide a light-emittingelement with low power consumption. Another object of one embodiment ofthe present invention is to provide a novel light-emitting element.Another object of one embodiment of the present invention is to providea novel light-emitting device. Another object of one embodiment of thepresent invention is to provide a novel display device.

Note that the description of the above object does not preclude theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

In one embodiment of the present invention, a light-emitting elementincludes a light-emitting layer in which an exciplex is formed, wherebytriplet excitons can be converted into singlet excitons and light can beemitted from the singlet excitons. The light-emitting element can emitlight from a fluorescent compound by utilizing energy transfer of thesinglet excitons.

Thus, one embodiment of the present invention is a light-emittingelement including a fluorescent material and a host material. The hostmaterial contains a first organic compound and a second organiccompound. The first organic compound and the second organic compound canform an exciplex. The proportion of a delayed fluorescence component inlight emitted from the exciplex is higher than or equal to 5%, and thedelayed fluorescence component contains a delayed fluorescence componentwhose fluorescence lifetime is 10 ns or longer and 50 μs or shorter.

In the above structure, the exciplex is preferably capable oftransferring excited energy to the fluorescent material.

In each of the above structures, light emitted from the exciplexpreferably has a region overlapping with an absorption band of thefluorescent material on the lowest energy side.

In each of the above structures, one of the first organic compound andthe second organic compound preferably has an electron transportingproperty, and the other of the first organic compound and the secondorganic compound preferably has a hole transporting property.Alternatively, one of the first organic compound and the second organiccompound preferably includes a π-electron deficient heteroaromatic ringskeleton, and the other of the first organic compound and the secondorganic compound preferably includes a π-electron rich heteroaromaticring skeleton or an aromatic amine skeleton.

Another embodiment of the present invention is a display deviceincluding the light-emitting element having any of the above-describedstructures, and at least one of a color filter and a transistor. Anotherembodiment of the present invention is an electronic device includingthe display device, and at least one of a housing and a touch sensor.Another embodiment of the present invention is a lighting deviceincluding the light-emitting element having any of the above-describedstructures, and at least one of a housing and a touch sensor. Thecategory of one embodiment of the present invention includes not onlythe light-emitting device including the light-emitting element but alsoan electronic device including the light-emitting device. Accordingly,the light-emitting device in this specification refers to an imagedisplay device and a light source (e.g., a lighting device). Thelight-emitting device may be included in a display module in which aconnector such as a flexible printed circuit (FPC) or a tape carrierpackage (TCP) is connected to a light-emitting device, a display modulein which a printed wiring board is provided on the tip of a TCP, or adisplay module in which an integrated circuit (IC) is directly mountedon a light-emitting element by a chip on glass (COG) method.

According to one embodiment of the present invention, a light-emittingelement that has high emission efficiency and contains a fluorescentcompound can be provided. According to another embodiment of the presentinvention, a light-emitting element with low power consumption can beprovided. According to another embodiment of the present invention, anovel light-emitting element can be provided. According to anotherembodiment of the present invention, a novel light-emitting device canbe provided. According to another embodiment of the present invention, anovel display device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects described above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention and FIG. 1C is adiagram illustrating the correlation of energy levels in alight-emitting layer;

FIGS. 2A and 2B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention and FIG. 2C is adiagram illustrating the correlation of energy levels in alight-emitting layer;

FIGS. 3A and 3B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention and FIG. 3C is adiagram illustrating the correlation of energy levels in alight-emitting layer;

FIGS. 4A and 4B are each a schematic cross-sectional view of alight-emitting element of one embodiment of the present invention;

FIGS. 5A and 5B are each a schematic cross-sectional view of alight-emitting element of one embodiment of the present invention;

FIGS. 6A to 6C are schematic cross-sectional views illustrating a methodfor manufacturing a light-emitting element of one embodiment of thepresent invention;

FIGS. 7A to 7C are schematic cross-sectional views illustrating a methodfor manufacturing a light-emitting element of one embodiment of thepresent invention;

FIGS. 8A and 8B are a top view and a schematic cross-sectional viewillustrating a display device of one embodiment of the presentinvention;

FIGS. 9A and 9B are schematic cross-sectional views each illustrating adisplay device of one embodiment of the present invention;

FIG. 10 is a schematic cross-sectional view illustrating a displaydevice of one embodiment of the present invention;

FIGS. 11A and 11B are schematic cross-sectional views each illustratinga display device of one embodiment of the present invention;

FIGS. 12A and 12B are schematic cross-sectional views each illustratinga display device of one embodiment of the present invention;

FIG. 13 is a schematic cross-sectional view illustrating a displaydevice of one embodiment of the present invention;

FIGS. 14A and 14B are schematic cross-sectional views each illustratinga display device of one embodiment of the present invention;

FIGS. 15A and 15B are a block diagram and a circuit diagram illustratinga display device of one embodiment of the present invention;

FIGS. 16A and 16B are circuit diagrams each illustrating a pixel circuitof a display device of one embodiment of the present invention;

FIGS. 17A and 17B are circuit diagrams each illustrating a pixel circuitof a display device of one embodiment of the present invention;

FIGS. 18A and 18B are perspective views of an example of a touch panelof one embodiment of the present invention;

FIGS. 19A to 19C are cross-sectional views of examples of a displaydevice and a touch sensor of one embodiment of the present invention;

FIGS. 20A and 20B are cross-sectional views each illustrating an exampleof a touch panel of one embodiment of the present invention;

FIGS. 21A and 21B are a block diagram and a timing chart of a touchsensor of one embodiment of the present invention;

FIG. 22 is a circuit diagram of a touch sensor of one embodiment of thepresent invention;

FIG. 23 is a perspective view of a display module of one embodiment ofthe present invention;

FIGS. 24A to 24G illustrate electronic devices of one embodiment of thepresent invention;

FIGS. 25A to 25C are a perspective view and cross-sectional viewsillustrating a light-emitting device of one embodiment of the presentinvention;

FIGS. 26A to 26D are cross-sectional views each illustrating alight-emitting device of one embodiment of the present invention;

FIGS. 27A to 27C illustrate a lighting device and an electronic deviceof one embodiment of the present invention;

FIG. 28 illustrates lighting devices of one embodiment of the presentinvention;

FIG. 29 shows current efficiency-luminance characteristics oflight-emitting elements in Example;

FIG. 30 shows external quantum efficiency-luminance characteristics oflight-emitting elements in Example;

FIG. 31 shows luminance-voltage characteristics of light-emittingelements in Example;

FIG. 32 shows electroluminescence spectra of light-emitting elements inExample;

FIG. 33 shows current efficiency-luminance characteristics oflight-emitting elements in Example;

FIG. 34 shows external quantum efficiency-luminance characteristics oflight-emitting elements in Example;

FIG. 35 shows luminance-voltage characteristics of light-emittingelements in Example;

FIG. 36 shows electroluminescence spectra of light-emitting elements inExample;

FIG. 37 shows emission spectra of thin films in Example;

FIG. 38 shows an absorption spectrum of a solution in Example; and

FIGS. 39A and 39B show results of time-resolved fluorescence measurementof thin films in Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings. However, the present invention is not limitedto description to be given below, and it is to be easily understood thatmodes and details thereof can be variously modified without departingfrom the purpose and the scope of the present invention. Accordingly,the present invention should not be interpreted as being limited to thecontent of the embodiments below.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for simplification. Therefore, the disclosedinvention is not necessarily limited to the position, the size, therange, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. Therefore,for example, description can be made even when “first” is replaced with“second” or “third”, as appropriate. In addition, the ordinal numbers inthis specification and the like are not necessarily the same as thosewhich specify one embodiment of the present invention.

In the description of modes of the present invention in thisspecification and the like with reference to the drawings, the samecomponents in different diagrams are commonly denoted by the samereference numeral in some cases.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other depending on the case or circumstances. Forexample, the term “conductive layer” can be changed into the term“conductive film” in some cases. Also, the term “insulating film” can bechanged into the term “insulating layer” in some cases.

In this specification and the like, a singlet excited state (S*) refersto a singlet state having excited energy. The lowest level of thesinglet excited energy level (S1 level) refers to the excited energylevel of the lowest singlet excited state. A triplet excited state (T*)refers to a triplet state having excited energy. The lowest level of thetriplet excited energy level (T1 level) refers to the excited energylevel of the lowest triplet excited state.

In this specification and the like, a fluorescent material or afluorescent compound refers to a material or a compound that emits lightin the visible light region when the relaxation from the singlet excitedstate to the ground state occurs. A phosphorescent material or aphosphorescent compound refers to a material or a compound that emitslight in the visible light region at room temperature when therelaxation from the triplet excited state to the ground state occurs.That is, a phosphorescent material or a phosphorescent compound refersto a material or a compound that can convert triplet excited energy intovisible light.

Note that in this specification and the like, “room temperature” refersto a temperature higher than or equal to 0° C. and lower than or equalto 40° C.

In this specification and the like, a wavelength range of blue refers toa wavelength range of greater than or equal to 400 nm and less than 490nm, and blue light has at least one peak in that wavelength range in anemission spectrum. A wavelength range of green refers to a wavelengthrange of greater than or equal to 490 nm and less than 580 nm, and greenlight has at least one peak in that wavelength range in an emissionspectrum. A wavelength range of red refers to a wavelength range ofgreater than or equal to 580 nm and less than or equal to 680 nm, andred light has at least one peak in that wavelength range in an emissionspectrum.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described below with reference to FIGS. 1A to1C.

<Structure Example of Light-Emitting Element>

First, a structure of the light-emitting element of one embodiment ofthe present invention will be described with reference to FIGS. 1A to1C.

FIG. 1A is a schematic cross-sectional view of a light-emitting element250 of one embodiment of the present invention.

The light-emitting element 250 includes a pair of electrodes (anelectrode 101 and an electrode 102) and an EL layer 100 between the pairof electrodes. The EL layer 100 includes at least a light-emitting layer130.

The EL layer 100 illustrated in FIG. 1A includes functional layers suchas a hole-injection layer 111, a hole-transport layer 112, anelectron-transport layer 118, and an electron-injection layer 119, inaddition to the light-emitting layer 130.

In this embodiment, although description is given assuming that theelectrode 101 and the electrode 102 of the pair of electrodes serve asan anode and a cathode, respectively, they are not limited thereto forthe structure of the light-emitting element 250. That is, the electrode101 may be a cathode, the electrode 102 may be an anode, and thestacking order of the layers between the electrodes may be reversed. Inother words, the hole-injection layer 111, the hole-transport layer 112,the light-emitting layer 130, the electron-transport layer 118, and theelectron-injection layer 119 may be stacked in this order from the anodeside.

The structure of the EL layer 100 is not limited to the structureillustrated in FIG. 1A, and a structure including at least one layerselected from the hole-injection layer 111, the hole-transport layer112, the electron-transport layer 118, and the electron-injection layer119 may be employed. Alternatively, the EL layer 100 may include afunctional layer which is capable of lowering a hole- orelectron-injection barrier, improving a hole- or electron-transportproperty, inhibiting a hole- or electron-transport property, orsuppressing a quenching phenomenon by an electrode, for example. Notethat the functional layers may each be a single layer or stacked layers.

FIG. 1B is a schematic cross-sectional view illustrating an example ofthe light-emitting layer 130 in FIG. 1A. The light-emitting layer 130 inFIG. 11B includes a host material 131 and a guest material 132. The hostmaterial 131 includes an organic compound 131_1 and an organic compound131_2.

The guest material 132 may be a light-emitting organic compound, and thelight-emitting organic compound is preferably a substance capable ofemitting fluorescence (hereinafter also referred to as a fluorescentcompound). A structure in which a fluorescent compound is used as theguest material 132 will be described below. The guest material 132 maybe referred to as the fluorescent material or the fluorescent compound.

In the light-emitting element 250 of one embodiment of the presentinvention, voltage application between the pair of electrodes (theelectrodes 101 and 102) allows electrons and holes to be injected fromthe cathode and the anode, respectively, into the EL layer 100 and thusa current flows. By recombination of the injected carriers (electronsand holes), excitons are formed. The ratio of singlet excitons totriplet excitons (hereinafter referred to as exciton generationprobability) which are generated by carrier (electrons and holes)recombination is approximately 1:3 according to the statisticallyobtained probability. Accordingly, in a light-emitting element that usesa fluorescent material, the probability of generation of singletexcitons, which contribute to light emission, is 25% and the probabilityof generation of triplet excitons, which do not contribute to lightemission, is 75%. Therefore, converting the triplet excitons, which donot contribute to light emission, into singlet excitons, whichcontribute to light emission, is important in increasing the emissionefficiency of the light-emitting element

<Light Emission Mechanism of Light-Emitting Element>

Next, the light emission mechanism of the light-emitting layer 130 isdescribed below.

The organic compound 131_1 and the organic compound 131_2 included inthe host material 131 in the light-emitting layer 130 form an exciplex.

Although it is acceptable as long as the combination of the organiccompound 131_1 and the organic compound 131_2 can form an exciplex, itis preferable that one of them be a compound having a function oftransporting holes (a hole-transport property) and the other be acompound having a function of transporting electrons (anelectron-transport property). In that case, a donor-acceptor exciplex isformed easily; thus, efficient formation of an exciplex is possible. Inthe case where the combination of the organic compounds 131_1 and 131_2is a combination of a compound having a hole-transport property and acompound having an electron-transport property, the carrier balance canbe easily controlled depending on the mixture ratio. Specifically, theweight ratio of the compound having a hole-transport property to thecompound having an electron-transport property is preferably within arange of 1:9 to 9:1. Since the carrier balance can be easily controlledwith the structure, a carrier recombination region can also becontrolled easily.

In order to efficiently form an exciplex, the combination of the hostmaterials preferably satisfies the follows the highest occupiedmolecular orbital (also referred to as HOMO) level of one of the organiccompound 131_1 and the organic compound 131_2 is higher than the HOMOlevel of the other of the organic compounds, and the lowest unoccupiedmolecular orbital (also referred to as LUMO) level of the one of theorganic compounds is higher than the LUMO level of the other of theorganic compounds. For example, when one of the organic compounds has ahole-transport property and the other of the organic compounds has anelectron-transport property, it is preferable that the HOMO level of theone of the organic compounds be higher than the HOMO level of the otherof the organic compounds and the LUMO level of the one of the organiccompounds be higher than the LUMO level of the other of the organiccompounds. Specifically, a difference in HOMO level between the organiccompounds is preferably greater than or equal to 0.05 eV, furtherpreferably greater than or equal to 0.1 eV, and still further preferablygreater than or equal to 0.2 eV A difference in LUMO level between theorganic compounds is preferably greater than or equal to 0.05 eV,further preferably greater than or equal to 0.1 eV, and still furtherpreferably greater than or equal to 0.2 eV.

FIG. 1C shows a correlation of energy levels of the organic compound131_1, the organic compound 131_2, and the guest material 132 in thelight-emitting layer 130. The following explains what terms and signs inFIG. 1C represent:

Host (131_1): the organic compound 131_1;

Host (131_2): the organic compound 131_2;

Guest (132): the guest material 132 (the fluorescent compound);

S_(H): the S1 level of the organic compound 131_1 (the host material);

T_(H): the T1 level of the organic compound 131_1 (the host material);

S_(G): the S1 level of the guest material 132 (the fluorescentcompound);

T_(G): the T1 level of the guest material 132 (the fluorescentcompound);

S_(E): the S1 level of the exciplex; and

T_(E): the T1 level of the exciplex.

In the light-emitting element of one embodiment of the presentinvention, the organic compounds 131_1 and 131_2 included in thelight-emitting layer 130 form an exciplex. The lowest singlet excitedenergy level of the exciplex (S_(E)) and the lowest triplet excitedenergy level of the exciplex (T_(E)) are adjacent to each other (seeRoute E₃ in FIG. 1C).

An exciplex is an excited state formed from two kinds of substances. Inphotoexcitation, the exciplex is formed by interaction between onesubstance in an excited state and the other substance in a ground state.The two kinds of substances that have formed the exciplex return to aground state by emitting light and then serve as the original two kindsof substances. In electrical excitation, when one substance is broughtinto an excited state, the one immediately interacts with the othersubstance to form an exciplex. Alternatively, one substance receives ahole and the other substance receives an electron to readily form anexciplex. In this case, any of the substances can form an exciplexwithout forming an excited state and; accordingly, most excitons in thelight-emitting layer 130 can exist as exciplexes. Because the excitedenergy levels of the exciplex (S_(E) and T_(E)) are lower than thesinglet excited energy level of the host materials (S_(H)) (the organiccompound 131_1 and the organic compound 131_2) that form the exciplex,the excited state of the host material 131 can be formed with lowerexcited energy. Accordingly, the driving voltage of the light-emittingelement 250 can be reduced.

Since the singlet excited energy level (S_(E)) and the triplet excitedenergy level (T_(E)) of the exciplex are adjacent to each other, theexciplex has a function of exhibiting thermally activated delayedfluorescence. In other words, the exciplex has a function of convertingtriplet excited energy to singlet excited energy by reverse intersystemcrossing (upconversion) (see Route E₄ in FIG. 1C). Thus, the tripletexcited energy generated in the light-emitting layer 130 is partlyconverted into singlet excited energy by the exciplex. In order to causethis conversion, the energy difference between the singlet excitedenergy level (S_(E)) and the triplet excited energy level (T_(E)) of theexciplex is preferably greater than 0 eV and less than or equal to 0.2eV. Note that in order to efficiently make reverse intersystem crossingoccur, the triplet excited energy level of the exciplex (T_(E)) ispreferably lower than the triplet excited energy levels of the organiccompounds (the organic compound 131_1 and the organic compound 131_2) inthe host material which form the exciplex. Thus, quenching of thetriplet excited energy of the exciplex due to the organic compounds isless likely to occur, which causes reverse intersystem crossingefficiently.

Furthermore, the singlet excited energy level of the exciplex (S_(E)) ispreferably higher than the singlet excited energy level of the guestmaterial 132 (S_(G)). In this way, the singlet excited energy of theformed exciplex can be transferred from the singlet excited energy levelof the exciplex (S_(E)) to the singlet excited energy level of the guestmaterial 132 (S_(G)), so that the guest material 132 is brought into thesinglet excited state, causing light emission (see Route E₅ in FIG. 1C).

To obtain efficient light emission from the singlet excited state of theguest material 132, the fluorescence quantum yield of the guest material132 is preferably high, and specifically, 50% or higher, furtherpreferably 70% or higher, still further preferably 90% or higher.

Note that since direct transition from a singlet ground state to atriplet excited state in the guest material 132 is forbidden, energytransfer from the singlet excited energy level of the exciplex (S_(E))to the triplet excited energy level of the guest material 132 (T_(G)) isunlikely to be a main energy transfer process.

When transfer of the triplet excited energy from the triplet excitedenergy level of the exciplex (T_(E)) to the triplet excited energy levelof the guest material 132 T_(G)) occurs, the triplet excited energy isdeactivated (see Route E₆ in FIG. 1C). Thus, it is preferable that theenergy transfer of Route E₆ be less likely to occur because theefficiency of generating the triplet excited state of the guest material132 can be decreased and thermal deactivation can be reduced. In orderto make this condition, the weight ratio of the guest material 132 tothe host material 131 is preferably low, specifically, preferablygreater than or equal to 0.001 and less than or equal to 0.05, furtherpreferably greater than or equal to 0.001 and less than or equal to0.01.

Note that when the direct carrier recombination process in the guestmaterial 132 is dominant, a large number of triplet excitons aregenerated in the light-emitting layer 130, resulting in decreasedemission efficiency due to thermal deactivation. Thus, it is preferablethat the probability of the energy transfer process through the exciplexformation process (Routes E₄ and E₅ in FIG. 1C) be higher than theprobability of the direct carrier recombination process in the guestmaterial 132 because the efficiency of generating the triplet excitedstate of the guest material 132 can be decreased and thermaldeactivation can be reduced. Therefore, as described above, the weightratio of the guest material 132 to the host material 131 is preferablylow, specifically, preferably greater than or equal to 0.001 and lessthan or equal to 0.05, further preferably greater than or equal to 0.001and less than or equal to 0.01.

By making all the energy transfer processes of Routes E₄ and E₅efficiently occur in the above-described manner, both the singletexcited energy and the triplet excited energy of the host material 131can be efficiently converted into the singlet excited energy of theguest material 132, whereby the light-emitting element 250 can emitlight with high emission efficiency.

The above-described processes through Routes E₃, E₄, and E₅ may bereferred to as exciplex-singlet energy transfer (ExSET) orexciplex-enhanced fluorescence (ExEF) in this specification and thelike. In other words, in the light-emitting layer 130, excited energy istransferred from the exciplex to the guest material 132.

When the light-emitting layer 130 has the above-described structure,light emission from the guest material 132 of the light-emitting layer130 can be obtained efficiently.

<Energy Transfer Mechanism>

Next, factors controlling the processes of intermolecular energytransfer between the host material 131 and the guest material 132 willbe described. As mechanisms of the intermolecular energy transfer, twomechanisms, i.e., Forster mechanism (dipole-dipole interaction) andDexter mechanism (electron exchange interaction), have been proposed.Although the intermolecular energy transfer process between the hostmaterial 131 and the guest material 132 is described here, the same canapply to a case where the host material 131 is an exciplex.

<<Förster Mechanism>>

In Förster mechanism, energy transfer does not require direct contactbetween molecules and energy is transferred through a resonantphenomenon of dipolar oscillation between the host material 131 and theguest material 132. By the resonant phenomenon of dipolar oscillation,the host material 131 provides energy to the guest material 132, andthus, the host material 131 in an excited state is brought to a groundstate and the guest material 132 in a ground state is brought to anexcited state. Note that the rate constant k_(h*→g) of Förster mechanismis expressed by Formula (1).

k h * → g = 9000 ⁢ c 4 ⁢ K 2 ⁢ ϕ ⁢ ln ⁢ 10 128 ⁢ π 5 ⁢ n 4 ⁢ N ⁢ τ ⁢ R 6 ⁢ ∫ f h ′( v ) ⁢ ε g ( v ) v 4 ⁢ dv ( 1 )

In Formula (1), ν denotes a frequency, f′_(h)(ν) denotes a normalizedemission spectrum of the host material 131 (a fluorescent spectrum inenergy transfer from a singlet excited state, and a phosphorescentspectrum in energy transfer from a triplet excited state), ε_(g)(ν)denotes a molar absorption coefficient of the guest material 132, Ndenotes Avogadro's number, n denotes a refractive index of a medium, Rdenotes an intermolecular distance between the host material 131 and theguest material 132, τ denotes a measured lifetime of an excited state(fluorescence lifetime or phosphorescence lifetime), c denotes the speedof light, ϕ denotes a luminescence quantum yield (a fluorescence quantumyield in energy transfer from a singlet excited state, and aphosphorescence quantum yield in energy transfer from a triplet excitedstate), and K² denotes a coefficient (0 to 4) of orientation of atransition dipole moment between the host material 131 and the guestmaterial 132. Note that K² is ⅔ in random orientation.

<<Dexter Mechanism>>

In Dexter mechanism, the host material 131 and the guest material 132are close to a contact effective range where their orbitals overlap, andthe host material 131 in an excited state and the guest material 132 ina ground state exchange their electrons, which leads to energy transfer.Note that the rate constant k_(h*→g) of Dexter mechanism is expressed byFormula (2).

k h * → g = ( 2 ⁢ π h ) ⁢ K 2 ⁢ exp ⁢ ( - 2 ⁢ R L ) ⁢ ∫ f h ′ ( v ) ⁢ ε g ′ ( v) ⁢ dv ( 2 )

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, ν denotes a frequency, f′_(h)(ν) denotes anormalized emission spectrum of the host material 131 (a fluorescentspectrum in energy transfer from a singlet excited state, and aphosphorescent spectrum in energy transfer from a triplet excitedstate), ε′_(g)(ν) denotes a normalized absorption spectrum of the guestmaterial 132, L denotes an effective molecular radius, and R denotes anintermolecular distance between the host material 131 and the guestmaterial 132.

Here, the efficiency of energy transfer from the host material 131 tothe guest material 132 (energy transfer efficiency ϕ_(ET)) is expressedby Formula (3). In the formula, k_(r) denotes a rate constant of alight-emission process (fluorescence in energy transfer from a singletexcited state, and phosphorescence in energy transfer from a tripletexcited state) of the host material 131, k_(n) denotes a rate constantof a non-light-emission process (thermal deactivation or intersystemcrossing) of the host material 131, and τ denotes a measured lifetime ofan excited state of the host material 131.

ϕ ET ⁢ k h * → g k r + k n + k h * → g = k h * → g ( 1 τ ) + k h * → g (3 )

According to Formula (3), it is found that the energy transferefficiency ϕ_(ET) can be increased by increasing the rate constantk_(h*→g) of energy transfer so that another competing rate constantk_(r)+k_(n)(=1/τ) becomes relatively small.

<<Concept for Promoting Energy Transfer>>

First, an energy transfer by Förster mechanism is considered. WhenFormula (1) is substituted into Formula (3), τ can be eliminated. Thus,in Förster mechanism, the energy transfer efficiency ϕ_(ET) does notdepend on the lifetime τ of the excited state of the host material 131.In addition, it can be said that the energy transfer efficiency ϕ_(ET)is higher when the luminescence quantum yield ϕ (here, the fluorescencequantum yield because energy transfer from a singlet excited state isdiscussed) is higher. In general, the luminescence quantum yield of anorganic compound in a triplet excited state is extremely low at roomtemperature. Thus, in the case where the host material 131 is in atriplet excited state, a process of energy transfer by Förster mechanismcan be ignored, and a process of energy transfer by Förster mechanism isconsidered only in the case where the host material 131 is in a singletexcited state.

Furthermore, it is preferable that the emission spectrum (thefluorescent spectrum in the case where energy transfer from a singletexcited state is discussed) of the host material 131 largely overlapwith the absorption spectrum (absorption corresponding to the transitionfrom the singlet ground state to the singlet excited state) of the guestmaterial 132. Moreover, it is preferable that the molar absorptioncoefficient of the guest material 132 be also high. This means that theemission spectrum of the host material 131 overlaps with the absorptionband of the guest material 132 which is on the longest wavelength side.Since direct transition from the singlet ground state to the tripletexcited state of the guest material 132 is forbidden, the molarabsorption coefficient of the guest material 132 in the triplet excitedstate can be ignored. Thus, a process of energy transfer to a tripletexcited state of the guest material 132 by Förster mechanism can beignored, and only a process of energy transfer to a singlet excitedstate of the guest material 132 is considered. That is, in Förstermechanism, a process of energy transfer from the singlet excited stateof the host material 131 to the singlet excited state of the guestmaterial 132 is considered.

Next, an energy transfer by Dexter mechanism is considered. According toFormula (2), in order to increase the rate constant k_(h*→g), it ispreferable that an emission spectrum of the host material 131 (afluorescent spectrum in the case where energy transfer from a singletexcited state is discussed) largely overlap with an absorption spectrumof the guest material 132 (absorption corresponding to transition from asinglet ground state to a singlet excited state). Therefore, the energytransfer efficiency can be optimized by making the emission spectrum ofthe host material 131 overlap with the absorption band of the guestmaterial 132 which is on the longest wavelength side.

When Formula (2) is substituted into Formula (3), it is found that theenergy transfer efficiency ϕ_(ET) in Dexter mechanism depends on τ. InDexter mechanism, which is a process of energy transfer based on theelectron exchange, as well as the energy transfer from the singletexcited state of the host material 131 to the singlet excited state ofthe guest material 132, energy transfer from the triplet excited stateof the host material 131 to the triplet excited state of the guestmaterial 132 occurs.

In the light-emitting element of one embodiment of the present inventionin which the guest material 132 is a fluorescent material, theefficiency of energy transfer to the triplet excited state of the guestmaterial 132 is preferably low. That is, the energy transfer efficiencybased on Dexter mechanism from the host material 131 to the guestmaterial 132 is preferably low and the energy transfer efficiency basedon Förster mechanism from the host material 131 to the guest material132 is preferably high.

As described above, the energy transfer efficiency in Förster mechanismdoes not depend on the lifetime τ of the excited state of the hostmaterial 131. In contrast, the energy transfer efficiency in Dextermechanism depends on the excitation lifetime τ of the host material 131.To reduce the energy transfer efficiency in Dexter mechanism, theexcitation lifetime τ of the host material 131 is preferably short.

In a manner similar to that of the energy transfer from the hostmaterial 131 to the guest material 132, the energy transfer by bothFörster mechanism and Dexter mechanism also occurs in the energytransfer process from the exciplex to the guest material 132.

Accordingly, one embodiment of the present invention provides alight-emitting element including, as the host material 131, the organiccompound 131_1 and the organic compound 131_2 which are a combinationfor forming an exciplex which functions as an energy donor capable ofefficiently transferring energy to the guest material 132. The exciplexformed by the organic compound 131_1 and the organic compound 131_2 hasa singlet excited energy level and a triplet excited energy level whichare adjacent to each other; accordingly, transition from a tripletexciton generated in the light-emitting layer 130 to a singlet exciton(reverse intersystem crossing) is likely to occur. This can increase theefficiency of generating singlet excitons in the light-emitting layer130. Furthermore, in order to facilitate energy transfer from thesinglet excited state of the exciplex to the singlet excited state ofthe guest material 132 having a function as an energy acceptor, it ispreferable that the emission spectrum of the exciplex overlap with theabsorption band of the guest material 132 which is on the longestwavelength side (lowest energy side). Thus, the efficiency of generatingthe singlet excited state of the guest material 132 can be increased.

In addition, fluorescence lifetime of a thermally activated delayedfluorescence component in light emitted from the exciplex is preferablyshort, and specifically, preferably 10 ns or longer and 50 μs orshorter, further preferably 10 ns or longer and 40 μs or shorter, stillfurther preferably 10 ns or longer and 30 μs or shorter.

The proportion of a thermally activated delayed fluorescence componentin the light emitted from the exciplex is preferably high. Specifically,the proportion of a thermally activated delayed fluorescence componentin the light emitted from the exciplex is preferably higher than orequal to 5%, further preferably higher than or equal to 8%, stillfurther preferably higher than or equal to 10%.

<Material>

Next, components of a light-emitting element of one embodiment of thepresent invention are described in detail below.

<<Light-Emitting Layer>>

Next, materials that can be used for the light-emitting layer 130 willbe described below.

In the light-emitting layer 130, the host material 131 is present in thelargest proportion by weight, and the guest material 132 (thefluorescent material) is dispersed in the host material 131. The S1level of the host material 131 (the organic compound 131_1 and theorganic compound 131_2) in the light-emitting layer 130 is preferablyhigher than the S1 level of the guest material 132 (the fluorescentmaterial) in the light-emitting layer 130. The T1 level of the hostmaterial 131 (the organic compound 131_1 and the organic compound 131_2)in the light-emitting layer 130 is preferably higher than the T1 levelof the guest material 132 (the fluorescent material) in thelight-emitting layer 130.

In the light-emitting layer 130, the guest material 132 is preferably,but not particularly limited to, an anthracene derivative, a tetracenederivative, a chrysene derivative, a phenanthrene derivative, a pyrenederivative, a perylene derivative, a stilbene derivative, an acridonederivative, a coumarin derivative, a phenoxazine derivative, aphenothiazine derivative, or the like, and for example, any of thefollowing materials can be used.

The examples include5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyidibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 6, coumarin 545T,N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrite(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene.

Examples of the organic compound 131_1 include a zinc- or aluminum-basedmetal complex, an oxadiazole derivative, a triazole derivative, abenzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxalinederivative, a dibenzothiophene derivative, a dibenzofuran derivative, apyrimidine derivative, a triazine derivative, a pyridine derivative, abipyridine derivative, a phenanthroline derivative, and the like. Otherexamples are an aromatic amine, a carbazole derivative, and the like.

Any of the following hole-transport materials and electron-transportmaterials can be used.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, anaromatic amine, a carbazole derivative, an aromatic hydrocarbon, astilbene derivative, or the like can be used. Furthermore, thehole-transport material may be a high molecular compound.

Examples of the material having a high hole-transport property areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivative are3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivative are4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbon are2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Other examples are pentacene, coronene, and the like. Thearomatic hydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or higherand having 14 to 42 carbon atoms is particularly preferable.

The aromatic hydrocarbon may have a vinyl skeleton. Examples of thearomatic hydrocarbon having a vinyl group are4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like.

Other examples are high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N,N′-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide]abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD).

Examples of the material having a high hole-transport property arearomatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation:TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),2-[N-(4-diphenylaminophenyl)-NV-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNRB),4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation:PCA1BP),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine(abbreviation: PCBiF),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF),N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF),N-[4-(9H-carbazol-9-yl)phenyl]-N-(4-phenyl)phenylaniline (abbreviation:YGA1BP), andN,N-bis[4-(carbazol-9-yl)phenyl]-N,N-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F). Other examples are amine compounds, carbazolecompounds, thiophene compounds, furan compounds, fluorene compounds;triphenylene compounds; phenanthrene compounds, and the like such as3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN),3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPPn), 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP),1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP),4-(3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl)dibenzofuran(abbreviation: mmDBFFLBi-II),4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-III), 1,3,5-tri(dibenzothiophen-4-yl)-benzene (abbreviated asDBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-vl)phenyl]dibenzothiophene(abbreviation: DBTFLP-II),4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV), and4-[3-(triphenylene-2-yl)phenyl]dibenzothiophene (abbreviation:mDBTPTp-II). The substances described here are mainly substances havinga hole mobility of 1×10⁻⁶ cm²/Vs or higher. Note that other than thesesubstances, any substance that has a property of transporting more holesthan electrons may be used.

As the electron-transport material, a material having a property oftransporting more electrons than holes can be used, and a materialhaving an electron mobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Aπ-electron deficient heteroaromatic compound such as anitrogen-containing heteroaromatic compound, a metal complex, or thelike can be used as the material which easily accepts electrons (thematerial having an electron-transport property). Specific examplesinclude a metal complex having a quinoline ligand, a benzoquinolineligand, an oxazole ligand, and a thiazole ligand. Other examples includean oxadiazole derivative, a triazole derivative, a phenanthrolinederivative, a pyridine derivative, a bipyridine derivative, a pyrimidinederivative, and the like.

Examples include metal complexes having a quinoline or benzoquinolineskeleton, such as tris(8-quinolinolato)aluminum(II) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq) and bis(8-quinolinolato)zinc(II) (abbreviation:Znq), and the like. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-benzoxazolyl)phenolate]zinc(II) (abbreviation: ZnPBO) orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ) can beused. Other than such metal complexes, any of the following can be used:heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:COl1), 3-(biphenyl-4-vl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI3),2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTB1m-II), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen); heterocyclic compounds having a diazine skeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBIPDBq),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 6mDBTPDBq-II),4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); heterocyclic compounds having a triazine skeleton such as2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn); heterocyclic compounds having a pyridineskeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine(abbreviation: 35DCzPPy); and heteroaromatic compounds such as4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Furtheralternatively, a high molecular compound such as poly(2,5-pyridinediyl)(abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances described here aremainly substances having an electron mobility of 1×10⁻⁶ cm²/Vs orhigher. Note that other substances may also be used as long as theirelectron-transport properties are higher than their hole-transportproperties.

As the organic compound 131_2, a substance which can form an exciplextogether with the organic compound 131_1 is used. Specifically, any ofthe hole-transport materials and the electron-transport materialsdescribed above can be used. In that case, it is preferable that theorganic compound 131_1, the organic compound 131_2, and the guestmaterial 132 (the fluorescent material) be selected such that theemission peak of the exciplex formed by the organic compound 131_1 andthe organic compound 131_2 overlaps with an absorption band on thelongest wavelength side (lowest energy side) of the guest material 132(the fluorescent material). This makes it possible to provide alight-emitting element with drastically improved emission efficiency.

As the host material 131 (the organic compound 131_1 and the organiccompound 131_2) included in the light-emitting layer 130, a materialhaving a function of converting triplet excited energy into singletexcited energy is preferable. As the material having a function ofconverting triplet excited energy into singlet excited energy, athermally activated delayed fluorescent (TADF) material can be given inaddition to the exciplex. Therefore, the term “exciplex” in thedescription can be replaced with the term “thermally activated delayedfluorescence material”. Note that the thermally activated delayedfluorescence material is a material having a small difference betweenthe triplet excited energy level and the singlet excited energy leveland a function of converting triplet excited energy into singlet excitedenergy by reverse intersystem crossing. Thus, the thermally activateddelayed fluorescence material can up-convert a triplet excited stateinto a singlet excited state (i.e., reverse intersystem crossing) usinga little thermal energy and efficiently exhibit light emission(fluorescence) from the singlet excited state. Thermally activateddelayed fluorescence is efficiently obtained under the condition wherethe difference between the triplet excited energy level and the singletexcited energy level is more than 0 eV and less than or equal to 0.2 eV,preferably more than 0 eV and less than or equal to 0.1 eV.

The material that exhibits thermally activated delayed fluorescence maybe a material that can form a singlet excited state by itself from atriplet excited state by reverse intersystem crossing. In the case wherethe thermally activated delayed fluorescence material is composed of onekind of material, any of the following materials can be used, forexample.

First, a fullerene, a derivative thereof, an acridine derivative such asproflavine, eosin, and the like can be given. Furthermore, ametal-containing porphyrin, such as a porphyrin containing magnesium(Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), orpalladium (Pd), can be given. Examples of the metal-containing porphyrininclude a protoporphyrin-tin fluoride complex (SnF₂(Proto IX)), amesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂(OEP)).

As the thermally activated delayed fluorescence material composed of onekind of material, a heterocyclic compound having a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring canbe used. Specifically,2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-d]carbazol-1-yl)-1,3,5-triazine(abbreviation: PIC-TRZ),2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTTn),2-[4-(10H-phenoxazin-10-yl)phenyl]-1,6-diphenyl-1,3,5-triazine(abbreviation: PXZ-TRZ),3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole(abbreviation: PPZ-3TPT),3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation:ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone(abbreviation: DMAC-DPS), or10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation:ACRSA), can be used. The heterocyclic compound is preferable because ofhaving the π-electron rich heteroaromatic ring and the π-electrondeficient heteroaromatic ring, for which the electron-transport propertyand the hole-transport property are high. Note that a substance in whichthe π-electron rich heteroaromatic ring is directly bonded to theπ-electron deficient heteroaromatic ring is particularly preferablebecause the donor property of the π-electron rich heteroaromatic ringand the acceptor property of the π-electron deficient heteroaromaticring are both increased and the difference between the singlet excitedlevel and the triplet excited level becomes small.

The light-emitting layer 130 can have a structure in which two or morelayers are stacked. For example, in the case where the light-emittinglayer 130 is formed by stacking a first light-emitting layer and asecond light-emitting layer in this order from the hole-transport layerside, the first light-emitting layer is formed using a substance havinga hole-transport property as the host material and the secondlight-emitting layer is formed using a substance having anelectron-transport property as the host material.

The light-emitting layer 130 may include a material other than the hostmaterial 131 and the guest material 132.

<<Pair of Electrodes>>

The electrode 101 and the electrode 102 have functions of injectingholes and electrons into the light-emitting layer 130. The electrode 101and the electrode 102 can be formed using a metal, an alloy, or aconductive compound, a mixture or a stack thereof, or the like. Atypical example of the metal is aluminum (Al); besides, a transitionmetal such as silver (Ag), tungsten, chromium, molybdenum, copper, ortitanium, an alkali metal such as lithium (Li) or cesium, or a Group 2metal such as calcium or magnesium (Mg) can be used. As the transitionmetal, a rare earth metal such as ytterbium (Yb) may be used. An alloycontaining any of the above metals can be used as the alloy, and MgAgand AlLi can be given as examples. Examples of the conductive compoundinclude metal oxides such as indium tin oxide (hereinafter, referred toas ITO), indium tin oxide containing silicon or silicon oxide (ITSO),indium zinc oxide, indium oxide containing tungsten and zinc, and thelike. It is also possible to use an inorganic carbon-based material suchas graphene as the conductive compound. As described above, theelectrode 101 and/or the electrode 102 may be formed by stacking two ormore of these materials.

Light emitted from the light-emitting layer 130 is extracted through theelectrode 101 and/or the electrode 102. Therefore, at least one of theelectrodes 101 and 102 transmits visible light. As the conductivematerial transmitting light, a conductive material having a visiblelight transmittance higher than or equal to 40% and lower than or equalto 100%, preferably higher than or equal to 60% and lower than or equalto 100%, and a resistivity lower than or equal to 1×10⁻² Ω·cm can beused. The electrode on the light extraction side may be formed using aconductive material having functions of transmitting light andreflecting light. As the conductive material, a conductive materialhaving a visible light reflectivity higher than or equal to 20% andlower than or equal to 80%, preferably higher than or equal to 40% andlower than or equal to 70%, and a resistivity lower than or equal to1×10⁻² Ω·cm can be used. In the case where the electrode through whichlight is extracted is formed using a material with low lighttransmittance, such as metal or alloy, the electrode 101 and/or theelectrode 102 is formed to a thickness that is thin enough to transmitvisible light (e.g., a thickness of 1 nm to 10 nm).

In this specification and the like, as the electrode transmitting light,a material that transmits visible light and has conductivity is used.Examples of the material include, in addition to the above-describedoxide conductor layer typified by an ITO, an oxide semiconductor layerand an organic conductor layer containing an organic substance Examplesof the organic conductive layer containing an organic substance includea layer containing a composite material in which an organic compound andan electron donor (donor material) are mixed and a layer containing acomposite material in which an organic compound and an electron acceptor(acceptor material) are mixed. The resistivity of the transparentconductive layer is preferably lower than or equal to 1×10⁵ Ω·cm,further preferably lower than or equal to 1×10⁴ Ω·cm.

As the method for forming the electrode 101 and the electrode 102, asputtering method, an evaporation method, a printing method, a coatingmethod, a molecular beam epitaxy (MBE) method, a CVD method, a pulsedlaser deposition method, an atomic layer deposition (ALD) method, or thelike can be used as appropriate.

<<Hole-Injection Layer>>

The hole-injection layer 111 has a function of reducing a barrier forhole injection from one of the pair of electrodes (the electrode 101 orthe electrode 102) to promote hole injection and is formed using atransition metal oxide, a phthalocyanine derivative, or an aromaticamine, for example. As the transition metal oxide, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be given. As the phthalocyanine derivative, phthalocyanine,metal phthalocyanine, or the like can be given. As the aromatic amine, abenzidine derivative, a phenylenediamine derivative, or the like can begiven. It is also possible to use a high molecular compound such aspolythiophene or polyaniline; a typical example thereof ispoly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which isself-doped polythiophene.

As the hole-injection layer 111, a layer containing a composite materialof a hole-transport material and a material having a property ofaccepting electrons from the hole-transport material can also be used.Alternatively, a stack of a layer containing a material having anelectron accepting property and a layer containing a hole-transportmaterial may also be used. In a steady state or in the presence of anelectric field, electric charge can be transferred between thesematerials. As examples of the material having an electron-acceptingproperty, organic acceptors such as a quinodimethane derivative, achloranil derivative, and a hexaazatriphenylene derivative can be given.A specific example is a compound having an electron-withdrawing group (ahalogen group or a cyano group), such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:Fa-TCNQ), chloranil, or2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN). Alternatively, a transition metal oxide such as an oxide of ametal from Group 4 to Group 8 can also be used. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, rhenium oxide, or the like can be used.In particular, molybdenum oxide is preferable because it is stable inthe air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, any ofthe aromatic amine, carbazole derivative, aromatic hydrocarbon, stilbenederivative, and the like described as examples of the hole-transportmaterial that can be used in the light-emitting layer 130 can be used.Furthermore, the hole-transport material may be a high molecularcompound.

<<Hole-Transport Layer>>

The hole-transport layer 112 is a layer containing a hole-transportmaterial and can be formed using any of the materials given as examplesof the material of the hole-injection layer 111. In order that thehole-transport layer 112 has a function of transporting holes injectedinto the hole-injection layer 111 to the light-emitting layer 130, theHOMO level of the hole-transport layer 112 is preferably equal or closeto the HOMO level of the hole-injection layer 111.

As the hole-transport material, any of the materials given as examplesof the material of the hole-injection layer 111 can be used. As thehole-transport material, a substance having a hole mobility of 1×10⁻⁶cm²/Vs or higher is preferably used. Note that any substance other thanthe above substances may be used as long as the hole-transport propertyis higher than the electron-transport property. The layer including asubstance having a high hole-transport property is not limited to asingle layer, and two or more layers containing the aforementionedsubstances may be stacked.

<<Electron-Transport Layer>>

The electron-transport layer 118 has a function of transporting, to thelight-emitting layer 130, electrons injected from the other of the pairof electrodes (the electrode 101 or the electrode 102) through theelectron-injection layer 119. A material having a property oftransporting more electrons than holes can be used as theelectron-transport material, and a material having an electron mobilityof 1×10⁻⁶ cm²/Vs or higher is preferable. As the compound which easilyaccepts electrons (the material having an electron-transport property),a π-electron deficient heteroaromatic compound such as anitrogen-containing heteroaromatic compound, a metal complex, or thelike can be used, for example. Specifically, a metal complex having aquinoline ligand, a benzoquinoline ligand, an oxazole ligand, or athiazole ligand, which are described as the electron-transport materialsthat can be used in the light-emitting layer 130, can be given. Further,an oxadiazole derivative; a triazole derivative, a phenanthrolinederivative, a pyridine derivative, a bipyridine derivative, a pyrimidinederivative, and the like can be given. A substance having an electronmobility of higher than or equal to 1×10⁻⁶ cm²/Vs is preferable. Notethat other than these substances, any substance that has a property oftransporting more electrons than holes may be used for theelectron-transport layer. The electron-transport layer 118 is notlimited to a single layer, and may include stacked two or more layerscontaining the aforementioned substances.

Between the electron-transport layer 118 and the light-emitting layer130, a layer that controls transfer of electron carriers may beprovided. This is a layer formed by addition of a small amount of asubstance having a high electron-trapping property to a material havinga high electron-transport property described above, and the layer iscapable of adjusting carrier balance by suppressing transfer of electroncarriers. Such a structure is very effective in preventing a problem(such as a reduction in element lifetime) caused when electrons passthrough the light-emitting layer.

<<Electron-Injection Layer>>

The electron-injection layer 119 has a function of reducing a barrierfor electron injection from the electrode 102 to promote electroninjection and can be formed using a Group 1 metal or a Group 2 metal, oran oxide, a halide, or a carbonate of any of the metals, for example.Alternatively, a composite material containing an electron-transportmaterial (described above) and a material having a property of donatingelectrons to the electron-transport material can also be used. As thematerial having an electron-donating property, a Group 1 metal, a Group2 metal, an oxide of any of the metals, or the like can be given.Specifically, an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiO_(x)), can be used. Alternatively,a rare earth metal compound like erbium fluoride (ErF₃) can be used.Electride may also be used for the electron-injection layer 119.Examples of the electride include a substance in which elections areadded at high concentration to calcium oxide-aluminum oxide. Theelectron-injection layer 119 can be formed using the substance that canbe used for the electron-transport layer 118.

A composite material in which an organic compound and an electron donor(donor) are mixed may also be used for the electron-injection layer 119.Such a composite material is excellent in an electron-injection propertyand an electron-transport property because electrons are generated inthe organic compound by the electron donor. In this case, the organiccompound is preferably a material that is excellent in transporting thegenerated electrons. Specifically, the above-listed substances forforming the electron-transport layer 118 (e.g., the metal complexes andheteroaromatic compounds) can be used, for example. As the electrondonor, a substance showing an electron-donating property with respect tothe organic compound may be used. Specifically, an alkali metal, analkaline earth metal, and a rare earth metal are preferable, andlithium, cesium, magnesium, calcium, erbium, and ytterbium are given. Inaddition, an alkali metal oxide or an alkaline earth metal oxide ispreferable, and lithium oxide, calcium oxide, barium oxide, and the likeare given. A Lewis base such as magnesium oxide can also be used. Anorganic compound such as tetrathiafulvalene (abbreviation: TTF) can alsobe used.

Note that the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer described above can each be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, a gravure printing method, or the like.Besides the above-mentioned materials, an inorganic compound such as aquantum dot or a high molecular compound (e.g., an oligomer, adendrimer, and a polymer) may be used in the light-emitting layer, thehole-injection layer, the hole-transport layer, the electron-transportlayer, and the electron-injection layer.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot,a core-shell quantum dot, or a core quantum dot, for example. Thequantum dot containing elements belonging to Groups 2 and 16, elementsbelonging to Groups 13 and 15, elements belonging to Groups 13 and 17,elements belonging to Groups 11 and 17, or elements belonging to Groups14 and 15 may be used. Alternatively, the quantum dot containing anelement such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S),phosphorus (P), indium (in), tellurium (Te), lead (Pb), gallium (Ga),arsenic (As), or aluminum (Al) may be used

<<Substrate>>

A light-emitting element in one embodiment of the present invention maybe formed over a substrate of glass, plastic, or the like. As the way ofstacking layers over the substrate, layers may be sequentially stackedfrom the electrode 101 side or sequentially stacked from the electrode102 side.

For the substrate over which the light-emitting element of oneembodiment of the present invention can be formed, glass, quartz,plastic, or the like can be used, for example. Alternatively, a flexiblesubstrate can be used. The flexible substrate means a substrate that canbe bent, such as a plastic substrate made of polycarbonate orpolyarylate, for example. Alternatively, a film, an inorganic vapordeposition film, or the like can be used. Another material may be usedas long as the substrate functions as a support in a manufacturingprocess of the light-emitting element or an optical element or as longas it has a function of protecting the light-emitting element or anoptical element.

In this specification and the like, a light-emitting element can beformed using any of a variety of substrates, for example. The type of asubstrate is not limited particularly. Examples of the substrate includea semiconductor substrate (e.g., a single crystal substrate or a siliconsubstrate), an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, paper including a fibrous material, a base material film, and thelike. As an example of a glass substrate, a barium borosilicate glasssubstrate, an aluminoborosilicate glass substrate, a soda lime glasssubstrate, and the like can be given. Examples of the flexiblesubstrate, the attachment film, the base material film, and the like aresubstrates of plastics typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polyether sulfone (PES), andpolytetrafluoroethylene (PTFE). Another example is a resin such asacrylic. Furthermore, polypropylene, polyester, polyvinyl fluoride, andpolyvinyl chloride can be given as examples. Other examples arepolyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film,paper, and the like.

Alternatively, a flexible substrate may be used as the substrate suchthat the light-emitting element is provided directly on the flexiblesubstrate. Further alternatively, a separation layer may be providedbetween the substrate and the light-emitting element. The separationlayer can be used when part or the whole of a light-emitting elementformed over the separation layer is separated from the substrate andtransferred onto another substrate. In such a case, the light-emittingelement can be transferred to a substrate having low heat resistance ora flexible substrate as well. For the above separation layer, a stackincluding inorganic films, which are a tungsten film and a silicon oxidefilm, and a structure in which a resin film of polyimide or the like isformed over a substrate can be used, for example.

In other words, after the light-emitting element is formed using asubstrate, the light-emitting element may be transferred to anothersubstrate. Example of the substrate to which the light-emitting elementis transferred are, in addition to the above substrates, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, and hemp), a syntheticfiber (e.g., nylon, polyurethane, and polyester), a regenerated fiber(e.g., acetate, cupra, rayon, and regenerated polyester), and the like),a leather substrate, a rubber substrate, and the like. When such asubstrate is used, a light-emitting element with high durability, highheat resistance, reduced weight, or reduced thickness can be formed.

The light-emitting element 250 may be formed over an electrodeelectrically connected to a field-effect transistor (FET), for example,which is formed over any of the above-described substrates. Accordingly,an active matrix display device in which the FET controls the driving ofthe light-emitting element can be manufactured.

In this embodiment, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inthe other embodiments. One embodiment of the present invention is notlimited to the above-described examples. In one embodiment of thepresent invention, an example where a light-emitting element contains afluorescent material and a host material and the host material containsa first organic compound and a second organic compound has beendescribed; however, one embodiment of the present invention is notlimited to this example. Depending on circumstances or conditions, inone embodiment of the present invention, the host material does notnecessarily contain the first organic compound or the second organiccompound. In addition, in one embodiment of the present invention, anexample where an exciplex contains a delayed fluorescence material whosefluorescence lifetime is 10 ns or longer and 50 μs or shorter has beendescribed; however, one embodiment of the present invention is notlimited to this example. Depending on circumstances or conditions, inone embodiment of the present invention, the exciplex may contain adelayed fluorescence material whose fluorescence lifetime is shorterthan 10 ns. Alternatively, the exciplex may contain a delayedfluorescence material whose fluorescence lifetime is longer than 50 μs.In addition, in one embodiment of the present invention, an examplewhere a delayed fluorescence component accounts for 5% or more ofemission of an exciplex is shown; however, one embodiment of the presentinvention is not limited thereto. Depending on circumstances orconditions, in one embodiment of the present invention, a delayedfluorescence component may account for less than 5% of emission of theexciplex.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 2

In this embodiment, light-emitting elements having structures differentfrom that described in Embodiment 1 and light emission mechanisms of thelight-emitting elements are described below with reference to FIGS. 2Ato 2C and FIGS. 3A to 3C. In FIGS. 2A to 2C and FIGS. 3A to 3C, aportion having a function similar to that in FIG. 1A is represented bythe same hatch pattern as in FIG. 1A and not especially denoted by areference numeral in some cases. In addition, common reference numeralsare used for portions having similar functions, and a detaileddescription of the portions is omitted in some cases.

<Structure Example 1 of Light-Emitting Element>

FIG. 2A is a schematic cross-sectional view of a light-emitting element260.

The light-emitting element 260 illustrated in FIG. 2A includes aplurality of light-emitting units (a light-emitting unit 106 and alight-emitting unit 108 in FIG. 2A) between a pair of electrodes (theelectrode 101 and the electrode 102). One light-emitting unit has thesame structure as the EL layer 100 illustrated in FIG. 1A. That is, thelight-emitting element 250 in FIG. 1A includes one light-emitting unit,while the light-emitting element 260 includes a plurality oflight-emitting units. Note that the electrode 101 functions as an anodeand the electrode 102 functions as a cathode in the followingdescription of the light-emitting element 260; however, the functionsmay be interchanged in the light-emitting element 260.

In the light-emitting element 260 illustrated in FIG. 2A, thelight-emitting unit 106 and the light-emitting unit 108 are stacked, anda charge-generation layer 115 is provided between the light-emittingunit 106 and the light-emitting unit 108. Note that the light-emittingunit 106 and the light-emitting unit 108 may have the same structure ordifferent structures. For example, it is preferable that the EL layer100 illustrated in FIG. 1A be used in the light-emitting unit 108.

The light-emitting element 260 includes a light-emitting layer 120 andthe light-emitting layer 130. The light-emitting unit 106 includes thehole-injection layer 111, the hole-transport layer 112, anelectron-transport layer 113, and an electron-injection layer 114 inaddition to the light-emitting layer 120. The light-emitting unit 108includes a hole-injection layer 116, a hole-transport layer 117, anelectron-transport layer 118, and an electron-injection layer 119 inaddition to the light-emitting layer 130.

The charge-generation layer 115 may have either a structure in which anacceptor substance that is an electron acceptor is added to ahole-transport material or a structure in which a donor substance thatis an electron donor is added to an electron-transport material.Alternatively, both of these structures may be stacked.

In the case where the charge-generation layer 115 contains a compositematerial of an organic compound and an acceptor substance, the compositematerial that can be used for the hole-injection layer 111 described inEmbodiment 1 may be used for the composite material. As the organiccompound, a variety of compounds such as an aromatic amine compound, acarbazole compound, an aromatic hydrocarbon, and a high molecularcompound (such as an oligomer, a dendrimer, or a polymer) can be used. Asubstance having a hole mobility of 1×10⁻⁶ cm²/Vs or higher ispreferably used as the organic compound. Note that any other materialmay be used as long as it has a property of transporting more holes thanelectrons. Since the composite material of an organic compound and anacceptor substance has excellent carrier-injection and carrier-transportproperties, low-voltage driving or low-current driving can be realized.Note that when a surface of a light-emitting unit on the anode side isin contact with the charge-generation layer 115 like the light-emittingunit 108, the charge-generation layer 115 can also serve as ahole-injection layer or a hole-transport layer of the light-emittingunit; thus, a hole-injection layer or a hole-transport layer need not beincluded in the light-emitting unit.

The charge-generation layer 115 may have a stacked structure of a layercontaining the composite material of an organic compound and an acceptorsubstance and a layer containing another material. For example, thecharge-generation layer 115 may be formed using a combination of a layercontaining the composite material of an organic compound and an acceptorsubstance with a layer containing one compound selected from amongelectron-donating materials and a compound having a highelectron-transport property. Furthermore, the charge-generation layer115 may be formed using a combination of a layer containing thecomposite material of an organic compound and an acceptor substance witha layer containing a transparent conductive material.

The charge-generation layer 115 provided between the light-emitting unit106 and the light-emitting unit 108 may have any structure as long aselectrons can be injected to the light-emitting unit on one side andholes can be injected into the light-emitting unit on the other sidewhen a voltage is applied between the electrode 101 and the electrode102. For example, in FIG. 2A, the charge-generation layer 115 injectselectrons into the light-emitting unit 106 and holes into thelight-emitting unit 108 when a voltage is applied such that thepotential of the electrode 101 is higher than that of the electrode 102.

Note that in terms of light extraction efficiency, the charge-generationlayer 115 preferably has a visible light transmittance (specifically, avisible light transmittance of higher than or equal to 40%). Thecharge-generation layer 115 functions even if it has lower conductivitythan the pair of electrodes (the electrodes 101 and 102). In the casewhere the conductivity of the charge-generation layer 115 is as high asthose of the pair of electrodes, carriers generated in thecharge-generation layer 115 flow toward the film surface direction, sothat light is emitted in a region where the electrode 101 and theelectrode 102 do not overlap, in some cases. To suppress such a defect,the charge-generation layer 115 is preferably formed using a materialwhose conductivity is lower than those of the pair of electrodes.

Note that forming the charge-generation layer 115 by using any of theabove materials can suppress an increase in drive voltage caused by thestack of the light-emitting layers.

The light-emitting element having two light-emitting units is describedwith reference to FIG. 2A; however, a similar structure can be appliedto a light-emitting element in which three or more light-emitting unitsare stacked. With a plurality of light-emitting units partitioned by thecharge-generation layer between a pair of electrodes as in thelight-emitting element 260, it is possible to provide a light-emittingelement which can emit light with high luminance with the currentdensity kept low and has a long lifetime. A light-emitting element withlow power consumption can be provided.

When the structure of the EL layer 100 illustrated in FIG. 1A is usedfor at least one of the plurality of units, a light-emitting elementwith high emission efficiency can be provided.

It is preferable that the light-emitting layer 130 included in thelight-emitting unit 108 have the structure described in Embodiment 1.Thus, the light-emitting element 260 contains a fluorescent material asa light-emitting material and has high luminous efficiency, which ispreferable.

Furthermore, the light-emitting layer 120 included in the light-emittingunit 108 contains a host material 121 and a guest material 122 asillustrated in FIG. 2B. Note that the guest material 122 is describedbelow as a fluorescent material.

<Light Emission Mechanism of Light-Emitting Layer 120>

The light emission mechanism of the light-emitting layer 120 isdescribed below.

By recombination of the electrons and holes injected from the pair ofelectrodes (the electrode 101 and the electrode 102) or thecharge-generation layer in the light-emitting layer 120, excitons areformed. Because the amount of the host material 121 is larger than thatof the guest material 122, the host material 121 is brought into anexcited state by the exciton generation.

Note that the term “exciton” refers to a carrier (electron and hole)pair. Since excitons have energy, a material where excitons aregenerated is brought into an excited state.

In the case where the formed excited state of the host material 121 is asinglet excited state, singlet excited energy transfers from the S1level of the host material 121 to the S1 level of the guest material122, thereby forming the singlet excited state of the guest material122.

Since the guest material 122 is a fluorescent material, when a singletexcited state is formed in the guest material 122, the guest material122 readily emits light. To obtain high emission efficiency in thiscase, the fluorescence quantum yield of the guest material 122 ispreferably high. The same can apply to a case where a singlet excitedstate is formed by recombination of carriers in the guest material 122.

Next, a case where recombination of carriers forms a triplet excitedstate of the host material 121 is described. The correlation of energylevels of the host material 121 and the guest material 122 in this caseis shown in FIG. 2C. The following explains what terms and signs in FIG.2C represent. Note that because it is preferable that the T1 level ofthe host material 121 be lower than the T1 level of the guest material122, FIG. 2C shows this preferable case. However, the T1 level of thehost material 121 may be higher than the T1 level of the guest material122.

Host (121): the host material 121;

Guest (122): the guest material 122 (the fluorescent material);

S_(FH): the S1 level the host material 121:

T_(FH): the T1 level of the host material 121;

S_(FG): the S1 level of the guest material 122 (the fluorescentmaterial); and

T_(FG): the T1 level of the guest material 122 (the fluorescentmaterial).

As illustrated in FIG. 2C, triplet excitons formed by carrierrecombination become adjacent to each other by triplet-tripletannihilation (TTA), a reaction in which one of the triplet excitons isconverted into a singlet exciton having energy of the S1 level of thehost material 121 (S_(FH)) is caused (see TTA in FIG. 2C). The singletexcited energy of the host material 121 is transferred from S_(FH) tothe S1 level of the guest material 122 (S_(FG)) having a lower energythan S_(FH) (see Route E₁ in FIG. 2C), and a singlet excited state ofthe guest material 122 is formed, whereby the guest material 122 emitslight.

Note that in the case where the density of triplet excitons in thelight-emitting layer 120 is sufficiently high (e.g., 1×10⁻¹² cm⁻³ ormore), only the reaction of two triplet excitons close to each other canbe considered whereas deactivation of a single triplet exciton can beignored.

In the case where a triplet excited state of the guest material 122 isformed by carrier recombination, the triplet excited state of the guestmaterial 122 is thermally deactivated and is difficult to use for lightemission. However, in the case where the T1 level of the host material121 (T_(FH)) is lower than the T1 level of the guest material 122(T_(FG)), the triplet excited energy of the guest material 122 can betransferred from the T1 level of the guest material 122 (T_(FG)) to theT1 level of the host material 121 (T_(FH)) (see Route E₂ in FIG. 2C) andthen is utilized for TTA.

In other words, the host material 121 preferably has a function ofconverting triplet excited energy into singlet excited energy by causingTTA, so that the triplet excited energy generated in the light-emittinglayer 120 can be partly converted into singlet excited energy by TTA inthe host material 121. The singlet excited energy can be transferred tothe guest material 122 and extracted as fluorescence. In order toachieve this, the S1 level of the host material 121 (S_(FH)) ispreferably higher than the S1 level of the guest material 122 (S_(FG)).In addition, the T1 level of the host material 121 (T_(FH)) ispreferably lower than the T1 level of the guest material 122 (T_(FG)).

Note that particularly in the case where the T1 level of the guestmaterial 122 (T_(GF)) is lower than the T1 level of the host material121 (T_(FH)), the weight ratio of the guest material 122 to the hostmaterial 121 is preferably low. Specifically, the weight ratio of theguest material 122 to the host material 121 is preferably greater than 0and less than or equal to 0.05, in which case, the probability ofcarrier recombination in the guest material 122 can be reduced. Inaddition, the probability of energy transfer from the T1 level of thehost material 121 (T_(FH)) to the T1 level of the guest material 122(T_(FG)) can be reduced.

Note that the host material 121 may be composed of a single compound ora plurality of compounds.

Note that in each of the above-described structures, the guest materials(fluorescent materials) used in the light-emitting unit 106 and thelight-emitting unit 108 may be the same or different. In the case wherethe same guest material is used for the light-emitting unit 106 and thelight-emitting unit 108, the light-emitting element 260 can exhibit highemission luminance at a small current value, which is preferable. In thecase where different guest materials are used for the light-emittingunit 106 and the light-emitting unit 108, the light-emitting element 260can exhibit multi-color light emission, which is preferable. It isparticularly favorable to select the guest materials so that white lightemission with high color rendering properties or light emission of atleast red, green, and blue can be obtained

<Structure Example 2 of Light-Emitting Element>

FIG. 3A is a schematic cross-sectional view of a light-emitting element262.

The light-emitting element 262 illustrated in FIG. 3A includes, like thelight-emitting element 260 described above, a plurality oflight-emitting units (a light-emitting unit 106 and a light-emittingunit 108 in FIG. 3A) between a pair of electrodes (the electrode 101 andthe electrode 102). One light-emitting unit has the same structure asthe EL layer 100 illustrated in FIG. 1A. Note that the light-emittingunit 106 and the light-emitting unit 108 may have the same structure ordifferent structures.

In the light-emitting element 262 illustrated in FIG. 3A, thelight-emitting unit 106 and the light-emitting unit 108 are stacked, anda charge-generation layer 115 is provided between the light-emittingunit 106 and the light-emitting unit 108. For example, it is preferablethat the EL layer 100 illustrated in FIG. 1A be used in thelight-emitting unit 106.

The light-emitting element 262 includes the light-emitting layer 130 anda light-emitting layer 140. The light-emitting unit 106 includes thehole-injection layer 111, the hole-transport layer 112, theelectron-transport layer 113, and the electron-injection layer 114 inaddition to the light-emitting layer 130. The light-emitting unit 108includes the hole-injection layer 116, the hole-transport layer 117, theelectron-transport layer 118, and the electron-injection layer 119 inaddition to the light-emitting layer 140.

In addition, the light-emitting layer of the light-emitting unit 108preferably contains a phosphorescent material. That is, it is preferablethat the light-emitting layer 130 included in the light-emitting unit106 have the structure described in Embodiment 1 and the light-emittinglayer 140 included in the light-emitting unit 108 contain aphosphorescent material. A structure example of the light-emittingelement 262 in this case is described below.

Furthermore, the light-emitting layer 140 included in the light-emittingunit 108 contains a host material 141 and a guest material 142 asillustrated in FIG. 3B. The host material 141 contains an organiccompound 141_1 and an organic compound 141_2. Note that the guestmaterial 142 included in the light-emitting layer 140 is described belowas a phosphorescent material.

<Light Emission Mechanism of Light-Emitting Layer 140>

Next, the light emission mechanism of the light-emitting layer 140 isdescribed. The organic compound 141_1 and the organic compound 141_2which are included in the light-emitting layer 140 form an exciplex.

It is acceptable as long as the combination of the organic compound141_1 and the organic compound 141_2 can form an exciplex in thelight-emitting layer 140, and it is preferred that one organic compoundhave a hole-transport property and the other organic compound have anelectron-transport property.

FIG. 3C illustrates the correlation of energy levels of the organiccompound 141_1, the organic compound 141_2, and the guest material 142in the light-emitting layer 140. The following explains what terms andsigns in FIG. 3C represent:

Host (141_1): the organic compound 141_1 (host material);

Host (141 2): the organic compound 141 2 (host material);

Guest (142): the guest material 142 (phosphorescent material);

S_(PH): the S1 level of the organic compound 141_1 (host material);

T_(PH): the T1 level of the organic compound 141 1 (host material);

T_(PG): the T1 level of the guest material 142 (phosphorescentmaterial);

S_(PF): the S1 level of the exciplex; and

T_(PE): the T1 level of the exciplex.

The level (S_(PE)) of the lowest singlet excited state of the exciplexformed by the organic compounds 141_1 and 141_2 and the level (T_(PE))of the lowest triplet excited state of the exciplex are close to eachother (see Route C in FIG. 3C).

Both energies of S_(PE) and T_(PE) of the exciplex are then transferredto the level of the lowest triplet excited state of the guest material142 (phosphorescent material); thus, light emission is obtained (seeRoute D in FIG. 3C).

The above-described processes through Route C and Route D may bereferred to as exciplex-triplet energy transfer (ExTET) in thisspecification and the like.

When one of the organic compounds 141_1 and 141_2 receives holes and theother receives electrons, the exciplex is formed. Alternatively, whenone compound is brought into an excited state, the one interacts withthe other compound to form the exciplex. Therefore, most excitons in thelight-emitting layer 140 exist as exciplexes. The band gap of theexciplex is narrower than that of each of the organic compounds 141_1and 141_2; therefore, an excited state can be formed with lowerexcitation energy. Thus, the formation of the exciplex can lower thedrive voltage of the light-emitting element.

When the light-emitting layer 140 has the above structure, lightemission from the guest material 142 (phosphorescent material) of thelight-emitting layer 140 can be efficiently obtained.

Note that light emitted from the light-emitting layer 130 preferably hasa peak on the shorter wavelength side than light emitted from thelight-emitting layer 140. Since the luminance of a light-emittingelement using a phosphorescent material emitting light with a shortwavelength tends to be degraded quickly, fluorescence with a shortwavelength is employed so that a light-emitting element with lessdegradation of luminance can be provided.

Furthermore, the light-emitting layer 130 and the light-emitting layer140 may be made to emit light with different emission wavelengths, sothat the light-emitting element can be a multicolor light-emittingelement. In that case, the emission spectrum of the light-emittingelement is formed by combining light having different emission peaks,and thus has at least two peaks.

The above structure is also suitable for obtaining white light emission.When the light-emitting layer 130 and the light-emitting layer 140 emitlight of complementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering propertythat is formed of three primary colors or four or more colors can beobtained by using a plurality of light-emitting materials emitting lightwith different wavelengths for one of the light-emitting layers 130 and140 or both. In that case, one of the light-emitting layers 130 and 140or both may be divided into layers and each of the divided layers maycontain a light-emitting material different from the others.

<Material that can be Used in Light-Emitting Layers>

Next, materials that can be used in the light-emitting layers 120, 130,and 140 are described.

<<Material that can be Used in Light-Emitting Layer 130>>

As a material that can be used in the light-emitting layer 130, amaterial that can be used in the light-emitting layer 130 in Embodiment1 may be used. Thus, a light-emitting element with high generationefficiency of a singlet excited state and high emission efficiency canbe fabricated.

<<Material that can be Used in Light-Emitting Layer 120>>

In the light-emitting layer 120, the host material 121 is present in thelargest proportion by weight, and the guest material 122 (thefluorescent material) is dispersed in the host material 121. The S1level of the host material 121 is preferably higher than the S1 level ofthe guest material 122 (the fluorescent material) while the T1 level ofthe host material 121 is preferably lower than the T1 level of the guestmaterial 122 (the fluorescent material).

In the light-emitting layer 120, although the guest material 122 is notparticularly limited, for example, any of materials which are describedas examples of the guest material 132 in Embodiment 1 can be used.

Although there is no particular limitation on a material that can beused as the host material 121 in the light-emitting layer 120, any ofthe following materials can be used, for example: metal complexes suchas tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BA1q), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: IAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP),2,9-bis(naphtthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline(abbreviation: NBphen), and9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:COl1); and aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). In addition, condensed polycyclic aromaticcompounds such as anthracene derivatives, phenanthrene derivatives,pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysenederivatives can be given, and specific examples are9,10-diphenylanthracene (abbreviation: DPAnth),N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),N,N-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetramine(abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-RuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3), and the like. One ormore substances having a wider energy gap than the guest material 122 ispreferably selected from these substances and known substances.

The light-emitting layer 120 can have a structure in which two or morelayers are stacked. For example, in the case where the light-emittinglayer 120 is formed by stacking a first light-emitting layer and asecond light-emitting layer in this order from the hole-transport layerside, the first light-emitting layer is formed using a substance havinga hole-transport property as the host material and the secondlight-emitting layer is formed using a substance having anelectron-transport property as the host material.

In the light-emitting layer 120, the host material 121 may be composedof one kind of compound or a plurality of compounds. Alternatively, thelight-emitting layer 120 may contain a material other than the hostmaterial 121 and the guest material 122.

<<Material that can be Used in Light-Emitting Layer 140>>

In the light-emitting layer 140, the host material 141 exists in thelargest proportion in weight ratio, and the guest material 142(phosphorescent material) is dispersed in the host material 141. The T1levels of the host materials 141 (organic compounds 141_1 and 141_2) ofthe light-emitting layer 140 are preferably higher than the T1 level ofthe guest material (guest material 142) of the light-emitting layer 140.

Examples of the organic compound 141 1 include a zinc- or aluminum-basedmetal complex, an oxadiazole derivative, a triazole derivative, abenzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxalinederivative, a dibenzothiophene derivative, a dibenzofuran derivative, apyrimidine derivative, a triazine derivative, a pyridine derivative, abipyridine derivative, a phenanthroline derivative, and the like Otherexamples are an aromatic amine, a carbazole derivative, and the like.Specifically, the electron-transport material and the hole-transportmaterial described in Embodiment 1 can be used.

As the organic compound 141_2, a substance which can form an exciplextogether with the organic compound 141_1 is preferably used.Specifically, the electron-transport material and the hole-transportmaterial described in Embodiment 1 can be used. In that case, it ispreferable that the organic compound 141_1, the organic compound 141_2,and the guest material 142 (phosphorescent material) be selected suchthat the emission peak of the exciplex formed by the organic compound141_1 and the organic compound 141_2 overlaps with an absorption band,specifically an absorption band on the longest wavelength side, of atriplet metal to ligand charge transfer (MLCT) transition of the guestmaterial 142 (phosphorescent material). This makes it possible toprovide a light-emitting element with drastically improved emissionefficiency. Note that in the case where a thermally activated delayedfluorescent material is used instead of the phosphorescent material, itis preferable that the absorption band on the longest wavelength side bea singlet absorption band.

As the guest material 142 (phosphorescent material), an iridium-,rhodium-, or platinum-based organometallic complex or metal complex canbe used; in particular, an organoiridium complex such as aniridium-based ortho-metalated complex is preferable. As anortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, animidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazineligand, an isoquinoline ligand, and the like can be given. As the metalcomplex, a platinum complex having a porphyrin ligand and the like canbe given.

Examples of the substance that has an emission peak in the blue or greenwavelength range include organometallic iridium complexes having a4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃),tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃), andtris[3-(5-biphenyl)-5-isopropyl-4-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPr5btz)₃); organometallic iridium complexes having a1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptz1-mp)₃) andtris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); organometallic iridium complexes havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃) andtris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and organometallic iridium complexes inwhich a phenylpyridine derivative having an electron-withdrawing groupis a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)), andbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)). Among the materials givenabove, the organometallic iridium complexes having a 4H-triazoleskeleton have high reliability and high emission efficiency and are thusespecially preferable.

Examples of the substance that has an emission peak in the green oryellow wavelength range include organometallic iridium complexes havinga pyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)-(acac)),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III)(abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)),(acetylacetonato)bis{4,6-dinmethyl-2-[6-(2,6-diminethylphenyl)-4-pyrimidinyl-κN3]phenyl-κC}iridium(III) (abbreviation: Ir(dmppm-dmp)₂(acac)),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes havinga pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); organometallic iridium complexeshaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation Ir(pq)₃),and bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); organometallic iridium complexes such asbis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)), andbis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)). Among the materials given above, the organometalliciridium complexes having a pyrimidine skeleton have distinctively highreliability and emission efficiency and are thus particularlypreferable.

Examples of the substance that has an emission peak in the yellow or redwavelength range include organometallic iridium complexes having apyrimidine skeleton, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methlylphenyl)pyrimidinato](dipivaloylmethanto)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), andbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(d1npm)₂(dpm)); organometallic iridium complexes havinga pyrazine skeleton, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)₂(dpm)), and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes havinga pyridine skeleton, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) andbis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)); a platinum complex such as2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and rare earth metal complexes such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) andtris[1-(2-thenoyl)-3,3,3-trifluoroacctonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). Among the materials given above, theorganometallic iridium complexes having a pyrimidine skeleton havedistinctively high reliability and emission efficiency and are thusparticularly preferable. Further, the organometallic iridium complexeshaving a pyrazine skeleton can provide red light emission with favorablechromaticity.

As the light-emitting material included in the light-emitting layer 140,any material can be used as long as the material can convert the tripletexcitation energy into light emission. As an example of the materialthat can convert the triplet excitation energy into light emission, athermally activated delayed fluorescent (TADF) material can be given inaddition to a phosphorescent material. Therefore, it is acceptable thatthe “phosphorescent material” in the description is replaced with the“thermally activated delayed fluorescence material” Note that thethermally activated delayed fluorescence material is a material having asmall difference between the triplet excited energy level and thesinglet excited energy level and a function of converting tripletexcitcd energy into singlet excitcd energy by reverse intersystemcrossing. Thus, the TADF material can up-convert a triplet excited stateinto a singlet excited state (i.e., reverse intersystem crossing ispossible) using a little thermal energy and efficiently exhibit lightemission (fluorescence) from the singlet excited state. The TADF isefficiently obtained under the condition where the difference in energybetween the triplet excited energy level and the singlet excited energylevel is preferably greater than 0 eV and less than or equal to 0.2 eV,further preferably greater than 0 eV and less than or equal to 0.1 eV.

The material that exhibits thermally activated delayed fluorescence maybe a material that can form a singlet excited state by itself from atriplet excited state by reverse intersystem crossing or may be acombination of a plurality of materials which form an exciplex.

In the case where the material exhibiting thermally activated delayedfluorescence is formed of one kind of material, any of the thermallyactivated delayed fluorescent materials described in Embodiment 1 can bespecifically used.

In the case where the thermally activated delayed fluorescent materialis used as the host material, it is preferable to use a combination oftwo kinds of compounds which form an exciplex. In this case, it isparticularly preferable to use the above-described combination of acompound which easily accepts electrons and a compound which easilyaccepts holes, which form an exciplex.

There is no limitation on the emission colors of the light-emittingmaterials contained in the light-emitting layers 120, 130, and 140, andthey may be the same or different. Light emitted from the light-emittingmaterials is mixed and extracted out of the element; therefore, forexample, in the case where their emission colors are complementarycolors, the light-emitting element can emit white light. Inconsideration of the reliability of the light-emitting element, theemission peak wavelength of the light-emitting material included in thelight-emitting layer 120 is preferably shorter than that of thelight-emitting material included in the light-emitting layer 140.

Note that the light-emitting units 106 and 108 and the charge-generationlayer 115 can be formed by an evaporation method (including a vacuumevaporation method), an ink-jet method, a coating method, gravureprinting, or the like.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, examples of light-emitting elements havingstructures different from those described in Embodiments 1 and 2 aredescribed below with reference to FIGS. 4A and 4B, FIGS. 5A and 5B,FIGS. 6A to 6C, and FIGS. 7A to 7C.

<Structure Example 1 of Light-Emitting Element>

FIGS. 4A and 4B are cross-sectional views each illustrating alight-emitting element of one embodiment of the present invention. InFIGS. 4A and 4B, a portion having a function similar to that in FIG. 1Ais represented by the same hatch pattern as in FIG. 1A and notespecially denoted by a reference numeral in some cases. In addition,common reference numerals are used for portions having similarfunctions, and a detailed description of the portions is omitted in somecases.

Light-emitting elements 270 a and 270 b in FIGS. 4A and 4B may have abottom-emission structure in which light is extracted through thesubstrate 200 or may have a top-emission structure in which lightemitted from the light-emitting element is extracted in the directionopposite to the substrate 200. However, one embodiment of the presentinvention is not limited to this structure, and a light-emitting elementhaving a dual-emission structure in which light emitted from thelight-emitting element is extracted in both top and bottom directions ofthe substrate 200 may be used.

In the case where the light-emitting elements 270 a and 270 b each havea bottom emission structure, the electrode 101 preferably has a functionof transmitting light and the electrode 102 preferably has a function ofreflecting light. Alternatively, in the case where the light-emittingelements 270 a and 270 b each have a top emission structure, theelectrode 101 preferably has a function of reflecting light and theelectrode 102 preferably has a function of transmitting light.

The light-emitting elements 270 a and 270 b each include the electrode101 and the electrode 102 over the substrate 200. Between the electrodes101 and 102, a light-emitting layer 123B, a light-emitting layer 123G,and a light-emitting layer 123R are provided. The hole-injection layer111, the hole-transport layer 112, the electron-transport layer 118, andthe electron-injection layer 119 are also provided.

The light-emitting element 270 b includes, as part of the electrode 101,a conductive layer 101 a, a conductive layer 101 b over the conductivelayer 101 a, and a conductive layer 101 c under the conductive layer 101a. In other words, the light-emitting element 270 b includes theelectrode 101 having a structure in which the conductive layer 101 a issandwiched between the conductive layer 101 b and the conductive layer101 c.

In the light-emitting element 270 b, the conductive layer 101 b and theconductive layer 101 c may be formed with different materials or thesame material. The electrode 101 preferably has a structure in which theconductive layer 101 a is sandwiched by the layers formed of the sameconductive material, in which case patterning by etching can beperformed easily.

In the light-emitting element 270 b, the electrode 101 may include oneof the conductive layer 101 b and the conductive layer 101 c.

For each of the conductive layers 101 a, 101 b, and 101 c, which areincluded in the electrode 101, the structure and materials of theelectrode 101 or 102 described in Embodiment 1 can be used.

In FIGS. 4A and 4B, a partition wall 145 is provided between a region221B, a region 221G, and a region 221R, which are sandwiched between theelectrode 101 and the electrode 102. The partition wall 145 has aninsulating property. The partition wall 145 covers end portions of theelectrode 101 and has openings overlapping with the electrode. With thepartition wall 145, the electrode 101 provided over the substrate 200 inthe regions can be divided into island shapes.

Note that the light-emitting layer 123B and the light-emitting layer123G may overlap with each other in a region where they overlap with thepartition wall 145. The light-emitting layer 123G and the light-emittinglayer 123R may overlap with each other in a region where they overlapwith the partition wall 145. The light-emitting layer 123R and thelight-emitting layer 123B may overlap with each other in a region wherethey overlap with the partition wall 145.

The partition wall 145 has an insulating property and is formed using aninorganic or organic material. Examples of the inorganic materialinclude silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, and aluminum nitride. Examples of theorganic material include photosensitive resin materials such as anacrylic resin and a polyimide resin.

The light-emitting layers 123R, 123G, and 123B preferably containlight-emitting materials having functions of emitting light of differentcolors. For example, when the light-emitting layer 123R contains alight-emitting material having a function of emitting red, the region221R emits red light. When the light-emitting layer 123G contains alight-emitting material having a function of emitting green, the region221G emits green light. When the light-emitting layer 123B contains alight-emitting material having a function of emitting blue, the region221B emits blue light. The light-emitting element 270 a or 270 b havingsuch a structure is used in a pixel of a display device, whereby afull-color display device can be fabricated. The thicknesses of thelight-emitting layers may be the same or different.

Any one or more of the light-emitting layers 123B, 123G, and 123Rpreferably include the light-emitting layer 130 described in Embodiment1, in which case a light-emitting element with high emission efficiencycan be fabricated.

One or more of the light-emitting layers 123B, 123G, and 123R mayinclude two or more stacked layers.

When at least one light-emitting layer includes the light-emitting layerdescribed in Embodiment 1 as described above and the light-emittingelement 270 a or 270 b including the light-emitting layer is used inpixels in a display device, a display device with high emissionefficiency can be fabricated. The display device including thelight-emitting element 270 a or 270 b can thus have reduced powerconsumption.

By providing a color filter over the electrode through which light isextracted, the color purity of each of the light-emitting elements 270 aand 270 b can be improved. Therefore, the color purity of a displaydevice including the light-emitting element 270 a or 270 b can beimproved.

By providing a polarizing plate over the electrode through which lightis extracted, the reflection of external light by each of thelight-emitting elements 270 a and 270 b can be reduced. Therefore, thecontrast ratio of a display device including the light-emitting element270 a or 270 b can be improved.

For the other components of the light-emitting elements 270 a and 270 b,the components of the light-emitting element in Embodiment 1 may bereferred to.

<Structure Example 2 of Light-Emitting Element>

Next, structure examples different from the light-emitting elementsillustrated in FIGS. 4A and 4B will be described below with reference toFIGS. 5A and 5B.

FIGS. 5A and 5B are cross-sectional views of a light-emitting element ofone embodiment of the present invention. In FIGS. 5A and 5B, a portionhaving a function similar to that in FIGS. 4A and 4B is represented bythe same hatch pattern as in FIGS. 4A and 4B and not especially denotedby a reference numeral in some cases. In addition, common referencenumerals are used for portions having similar functions, and a detaileddescription of such portions is not repeated in some cases.

FIGS. 5A and 5B illustrate structure examples of a light-emittingelement including the light-emitting layer between a pair of electrodes.A light-emitting element 272 a illustrated in FIG. 5A has a top-emissionstructure in which light is extracted in a direction opposite to thesubstrate 200, and a light-emitting element 272 b illustrated in FIG. 5Bhas a bottom-emission structure in which light is extracted to thesubstrate 200 side. However, one embodiment of the present invention isnot limited to these structures and may have a dual-emission structurein which light emitted from the light-emitting element is extracted inboth top and bottom directions with respect to the substrate 200 overwhich the light-emitting element is formed.

The light-emitting elements 272 a and 272 b each include the electrode101, the electrode 102, an electrode 103, and an electrode 104 over thesubstrate 200. At least a light-emitting layer 130 and thecharge-generation layer 115 are provided between the electrode 101 andthe electrode 102, between the electrode 102 and the electrode 103, andbetween the electrode 102 and the electrode 104. The hole-injectionlayer 111, the hole-transport layer 112, a light-emitting layer 150, theelectron-transport layer 113, the electron-injection layer 114, thehole-injection layer 116, the hole-transport layer 117, theelectron-transport layer 118, and the electron-injection layer 119 arefurther provided.

The electrode 101 includes a conductive layer 101 a and a conductivelayer 101 b over and in contact with the conductive layer 101 a Theelectrode 103 includes a conductive layer 103 a and a conductive layer103 b over and in contact with the conductive layer 103 a. The electrode104 includes a conductive layer 104 a and a conductive layer 104 b overand in contact with the conductive layer 104 a.

The light-emitting element 272 a illustrated in FIG. 5A and thelight-emitting element 272 b illustrated in FIG. 5B each include apartition wall 145 between a region 222B sandwiched between theelectrode 101 and the electrode 102, a region 222G sandwiched betweenthe electrode 102 and the electrode 103, and a region 222R sandwichedbetween the electrode 102 and the electrode 104. The partition wall 145has an insulating property. The partition wall 145 covers end portionsof the electrodes 101, 103, and 104 and has openings overlapping withthe electrodes With the partition wall 145, the electrodes provided overthe substrate 200 in the regions can be separated into island shapes.

The light-emitting elements 272 a and 272 b each include a substrate 220provided with an optical element 224B, an optical element 224G, and anoptical element 224R in the direction in which light emitted from theregion 222B, light emitted from the region 222G, and light emitted fromthe region 222R are extracted. The light emitted from each region isemitted outside the light-emitting element through each optical element.In other words, the light from the region 222B, the light from theregion 222G, and the light from the region 222R are emitted through theoptical element 224B, the optical element 224G, and the optical element224R, respectively.

The optical elements 224B, 224G, and 224R each have a function ofselectively transmitting light of a particular color out of incidentlight. For example, the light emitted from the region 222B through theoptical element 224B is blue light, the light emitted from the region222G through the optical element 224G is green light, and the lightemitted from the region 222R through the optical element 224R is redlight.

For example, a coloring layer (also referred to as color filter), a handpass filter, a multilayer filter, or the like can be used for theoptical elements 224R, 224G, and 224B. Alternatively, color conversionelements can be used as the optical elements. A color conversion elementis an optical element that converts incident light into light having alonger wavelength than the incident light. As the color conversionelements, quantum-dot elements can be favorably used. The usage of thequantum-dot type can increase color reproducibility of the displaydevice.

A plurality of optical elements may also be stacked over each of theoptical elements 224R, 224G, and 224B. As another optical element, acircularly polarizing plate, an anti-reflective film, or the like can beprovided, for example. A circularly polarizing plate provided on theside where light emitted from the light-emitting element of the displaydevice is extracted can prevent a phenomenon in which light enteringfrom the outside of the display device is reflected inside the displaydevice and returned to the outside. An anti-reflective film can weakenexternal light reflected by a surface of the display device. This leadsto clear observation of light emitted from the display device.

Note that in FIGS. 5A and 5B, blue light (B), green light (G), and redlight (R) emitted from the regions through the optical elements areschematically illustrated by arrows of dashed lines.

A light-blocking layer 223 is provided between the optical elements. Thelight-blocking layer 223 has a function of blocking light emitted fromthe adjacent regions. Note that a structure without the light-blockinglayer 223 may also be employed.

The light-blocking layer 223 has a function of reducing the reflectionof external light. The light-blocking layer 223 has a function ofpreventing mixture of light emitted from an adjacent light-emittingelement. As the light-blocking layer 223, a metal, a resin containingblack pigment, carbon black, a metal oxide, a composite oxide containinga solid solution of a plurality of metal oxides, or the like can beused.

For the substrate 200 and the substrate 220 provided with the opticalelements, the substrate in Embodiment 1 may be referred to.

Furthermore, the light-emitting elements 272 a and 272 b have amicrocavity structure.

Light emitted from the light-emitting layer 130 and the light-emittinglayer 150 resonates between a pair of electrodes (e.g., the electrode101 and the electrode 102). The light-emitting layer 130 and thelight-emitting layer 150 are formed at such a position as to intensifythe light of a desired wavelength among light to be emitted. Forexample, by adjusting the optical length from a reflective region of theelectrode 101 to the light-emitting region of the light-emitting layer130 and the optical length from a reflective region of the electrode 102to the light-emitting region of the light-emitting layer 130, the lightof a desired wavelength among light emitted from the light-emittinglayer 130 can be intensified. By adjusting the optical length from thereflective region of the electrode 101 to the light-emitting region ofthe light-emitting layer 150 and the optical length from the reflectiveregion of the electrode 102 to the light-emitting region of thelight-emitting layer 150, the light of a desired wavelength among lightemitted from the light-emitting layer 150 can be intensified. In thecase of a light-emitting element in which a plurality of light-emittinglayers (here, the light-emitting layers 130 and 150) are stacked, theoptical lengths of the light-emitting layers 130 and 150 are preferablyoptimized.

In each of the light-emitting elements 272 a and 272 b, by adjusting thethicknesses of the conductive layers (the conductive layer 101 b, theconductive layer 103 b, and the conductive layer 104 b) in each region,the light of a desired wavelength among light emitted from thelight-emitting layers 130 and 150 can be increased. Note that thethickness of at least one of the hole-injection layer 111 and thehole-transport layer 112 may differ between the regions to increase thelight emitted from the light-emitting layers 130 and 150.

For example, in the case where the refractive index of the conductivematerial having a function of reflecting light in the electrodes 101 to104 is lower than the refractive index of the light-emitting layer 130or 150, the thickness of the conductive layer 101 b of the electrode 101is adjusted so that the optical length between the electrode 101 and theelectrode 102 is m_(B)λ_(B)/2 (m_(B) is a natural number and λ_(B) isthe wavelength of light intensified in the region 2223). Similarly, thethickness of the conductive layer 103 b of the electrode 103 is adjustedso that the optical length between the electrode 103 and the electrode102 is m_(G)λ_(G)/2 (m_(G) is a natural number and λ_(G) is thewavelength of light intensified in the region 222G). Furthermore, thethickness of the conductive layer 104 b of the electrode 104 is adjustedso that the optical length between the electrode 104 and the electrode102 is m_(R)λ_(R)/2 (m_(R) is a natural number and λ_(R) is thewavelength of light intensified in the region 222R).

In the above manner, with the microcavity structure, in which theoptical length between the pair of electrodes in the respective regionsis adjusted, scattering and absorption of light in the vicinity of theelectrodes can be suppressed, resulting in high light extractionefficiency. In the above structure, the conductive layers 101 b, 103 b,and 104 b preferably have a function of transmitting light. Thematerials of the conductive layers 101 b, 103 b, and 104 b may be thesame or different. Each of the conductive layers 101 b, 103 b, and 104 bmay have a stacked structure of two or more layers.

Since the light-emitting element 272 a illustrated in FIG. 5A has atop-emission structure, it is preferable that the conductive layer 101a, the conductive layer 103 a, and the conductive layer 104 a have afunction of reflecting light. In addition, it is preferable that theelectrode 102 have functions of transmitting light and reflecting light.

Since the light-emitting element 272 b illustrated in FIG. 5B has abottom-emission structure, it is preferable that the conductive layer101 a, the conductive layer 103 a, and the conductive layer 104 a havefunctions of transmitting light and reflecting light. In addition, it ispreferable that the electrode 102 have a function of reflecting light.

In each of the light-emitting elements 272 a and 272 b, the conductivelayers 101 a, 103 a, and 104 a may be formed of different materials orthe same material. When the conductive layers 101 a. 103 a, and 104 aare formed of the same material, manufacturing cost of thelight-emitting elements 272 a and 272 b can be reduced. Note that eachof the conductive layers 101 a, 103 a, and 104 a may have a stackedstructure including two or more layers.

The light-emitting layer 130 in the light-emitting elements 272 a and272 b preferably has the structure described in Embodiment 1, in whichcase light-emitting elements with high emission efficiency can befabricated.

Either or both of the light-emitting layers 130 and 150 may have astacked structure of two layers, like a light-emitting layer 150 a and alight-emitting layer 150 b. The two light-emitting layers including twokinds of light-emitting materials (a first light-emitting material and asecond light-emitting material) for emitting different colors of lightenable light emission of a plurality of colors. It is particularlypreferable to select the light-emitting materials of the light-emittinglayers so that white light can be obtained by combining light emissionsfrom the light-emitting layers 130 and 150.

Either or both of the light-emitting layers 130 and 150 may have astacked structure of three or more layers, in which a layer notincluding a light-emitting material may be included.

In the above-described manner, the light-emitting element 272 a or 272 bincluding the light-emitting laver which has the structure described inEmbodiment 1 is used in pixels in a display device, whereby a displaydevice with high emission efficiency can be fabricated. Accordingly, thedisplay device including the light-emitting element 272 a or 272 b canhave low power consumption.

For the other components of the light-emitting elements 272 a and 272 b,the components of the light-emitting element 270 a or 270 b or thelight-emitting element in Embodiment 1 or 2 may be referred to.

<Fabrication Method of Light-Emitting Element>

Next, a method for fabricating a light-emitting element of oneembodiment of the present invention is described below with reference toFIGS. 6A to 6C and FIGS. 7A to 7C. Here, a method for fabricating thelight-emitting element 272 a illustrated in FIG. 5A is described.

FIGS. 6A to 6C and FIGS. 7A to 7C are cross-sectional views illustratinga method for fabricating the light-emitting element of one embodiment ofthe present invention.

The method for manufacturing the light-emitting element 272 a describedbelow includes first to seventh steps.

<<First Step>>

In the first step, the electrodes (specifically the conductive layer 101a of the electrode 101, the conductive layer 103 a of the electrode 103,and the conductive layer 104 a of the electrode 104) of thelight-emitting elements are formed over the substrate 200 (see FIG. 6A).

In this embodiment, a conductive layer having a function of reflectinglight is formed over the substrate 200 and processed into a desiredshape; whereby the conductive layers 101 a, 103 a, and 104 a are formed.As the conductive layer having a function of reflecting light, an alloyfilm of silver, palladium, and copper (also referred to as an Ag—Pd—Cufilm and APC) is used. The conductive layers 101 a, 103 a, and 104 a arepreferably formed through a step of processing the same conductivelayer, because the manufacturing cost can be reduced.

Note that a plurality of transistors may be formed over the substrate200 before the first step. The plurality of transistors may beelectrically connected to the conductive layers 101 a, 103 a, and 104 a.

<<Second Step>>

In the second step, the transparent conductive layer 101 b having afunction of transmitting light is formed over the conductive layer 101 aof the electrode 101, the transparent conductive layer 103 b having afunction of transmitting light is formed over the conductive layer 103 aof the electrode 103, and the transparent conductive layer 104 b havinga function of transmitting light is formed over the conductive layer 104a of the electrode 104 (see FIG. 61B).

In this embodiment, the conductive layers 101 b, 103 b, and 104 b eachhaving a function of transmitting light are formed over the conductivelayers 101 a, 103 a, and 104 a each having a function of reflectinglight, respectively, whereby the electrode 101, the electrode 103, andthe electrode 104 are formed. As the conductive layers 101 b, 103 b, and104 b, ITSO films are used.

The conductive layers 101 b, 103 b, and 104 b having a function oftransmitting light may be formed through a plurality of steps. When theconductive layers 101 b, 103 b, and 104 b having a function oftransmitting light are formed through a plurality of steps, they can beformed to have thicknesses which enable microcavity structuresappropriate in the respective regions.

<<Third Step>>

In the third step, the partition wall 145 that covers end portions ofthe electrodes of the light-emitting element is formed (see FIG. 6C).

The partition wall 145 includes an opening overlapping with theelectrode. The conductive film exposed by the opening functions as theanode of the light-emitting element. As the partition wall 145, apolyimide-based resin is used in this embodiment.

In the first to third steps, since there is no possibility of damagingthe EL layer (a layer containing an organic compound), a variety of filmformation methods and fine processing technologies can be employed. Inthis embodiment, a reflective conductive layer is formed by a sputteringmethod, a pattern is formed over the conductive layer by a lithographymethod, and then the conductive layer is processed into an island shapeby a dry etching method or a wet etching method to form the conductivelayer 101 a of the electrode 101, the conductive layer 103 a of theelectrode 103, and the conductive layer 104 a of the electrode 104.Then, a transparent conductive film is formed by a sputtering method, apattern is formed over the transparent conductive film by a lithographymethod, and then the transparent conductive film is processed intoisland shapes by a wet etching method to form the electrodes 101, 103,and 104.

<<Fourth Step>>

In the fourth step, the hole-injection layer 111, the hole-transportlayer 112, the light-emitting layer 150, the electron-transport layer113, the electron-injection layer 114, and the charge-generation layer115 are formed (see FIG. 7A).

The hole-injection layer 111 can be formed by co-evaporating ahole-transport material and a material containing an acceptor substance.Note that a co-evaporation method is an evaporation method in which aplurality of different substances is concurrently vaporized fromrespective different evaporation sources. The hole-transport layer 112can be formed by evaporating a hole-transport material.

The light-emitting layer 150 can be formed by evaporating the guestmaterial that emits light of at least one of green, yellow green,yellow, orange, and red. As the guest material, a fluorescent orphosphorescent organic compound can be used. In addition, thelight-emitting layer having any of the structures described inEmbodiments 1 and 2 is preferably used. The light-emitting layer 150 mayhave a two-layer structure. In that case, the two light-emitting layerspreferably contain light-emitting substances that emit light ofdifferent colors.

The electron-transport layer 113 can be formed by evaporating asubstance having a high electron-transport property. Theelectron-injection layer 114 can be formed by evaporating a substancehaving a high electron-injection property.

The charge-generation layer 115 can be formed by evaporating a materialobtained by adding an electron acceptor (acceptor) to a hole-transportmaterial or a material obtained by adding an electron donor (donor) toan electron-transport material.

<<Fifth Step>>

In the fifth step, the hole-injection layer 116, the hole-transportlayer 117, the light-emitting layer 130, the electron-transport layer118, the electron-injection layer 119, and the electrode 102 are formed(see FIG. 7B).

The hole-injection layer 116 can be formed by using a material and amethod which are similar to those of the hole-injection layer 111. Thehole-transport layer 117 can be formed by using a material and a methodwhich are similar to those of the hole-transport layer 112.

The light-emitting layer 130 can be formed by evaporating the guestmaterial that emits light of at least one color selected from violet,blue, and blue green. As the guest material, a fluorescent organiccompound can be used. The fluorescent organic compound may be evaporatedalone or the fluorescent organic compound mixed with another materialmay be evaporated. For example, the fluorescent organic compound may beused as a guest material, and the guest material may be dispersed into ahost material having higher excitation energy than the guest material.

The electron-transport layer 118 can be formed by using a material and amethod which are similar to those of the electron-transport layer 113.The electron-injection layer 119 can be formed by using a material and amethod which are similar to those of the electron-injection layer 114.

The electrode 102 can be formed by stacking a reflective conductive filmand a light-transmitting conductive film. The electrode 102 may have asingle-layer structure or a stacked-layer structure.

Through the above-described steps, the light-emitting element includingthe region 222B, the region 222G, and the region 222R over the electrode101, the electrode 103, and the electrode 104, respectively, are formedover the substrate 200.

<<Sixth Step>>

In the sixth step, the light-blocking layer 223, the optical element224B, the optical element 224G, and the optical element 224R are formedover the substrate 220 (see FIG. 7C).

As the light-blocking layer 223, a resin film containing black pigmentis formed in a desired region. Then, the optical element 224B, theoptical element 224G, and the optical element 224R are formed over thesubstrate 220 and the light-blocking layer 223. As the optical element224B, a resin film containing blue pigment is formed in a desiredregion. As the optical element 224G, a resin film containing greenpigment is formed in a desired region. As the optical element 224R, aresin film containing red pigment is formed in a desired region.

<<Seventh Step>>

In the seventh step, the light-emitting element formed over thesubstrate 200 is attached to the light-blocking layer 223, the opticalelement 2241, the optical element 224G, and the optical element 224Rformed over the substrate 220, and sealed with a sealant (notillustrated).

Through the above-described steps, the light-emitting element 272 aillustrated in FIG. 5A can be formed.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a display device of one embodiment of the presentinvention will be described below with reference to FIGS. 8A and 8B,FIGS. 9A and 9B, FIG. 10, FIGS. 11A and 11B, FIGS. 12A and 12B, FIG. 13,and FIGS. 14A and 14B.

<Structure Example 1 of Display Device>

FIG. 8A is a top view illustrating a display device 600 and FIG. 8B is across-sectional view taken along the dashed-dotted line A-B and thedashed-dotted line C-D in FIG. 8A. The display device 600 includesdriver circuit portions (a signal line driver circuit portion 601 and ascan line driver circuit portion 603) and a pixel portion 602. Note thatthe signal line driver circuit portion 601, the scan line driver circuitportion 603, and the pixel portion 602 have a function of controllinglight emission of a light-emitting element.

The display device 600 also includes an element substrate 610, a sealingsubstrate 604, a sealant 605, a region 607 surrounded by the sealant605, a lead wiring 608, and an FPC 609.

Note that the lead wiring 608 is a wiring for transmitting signals to beinput to the signal line driver circuit portion 601 and the scan linedriver circuit portion 603 and for receiving a video signal, a clocksignal, a start signal, a reset signal, and the like from the FPC 609serving as an external input terminal. Although only the FPC 609 isillustrated here, the FPC 609 may be provided with a printed wiringboard (PWB).

As the signal line driver circuit portion 601, a CMOS circuit in whichan n-channel transistor 623 and a p-channel transistor 624 are combinedis formed. As the signal line driver circuit portion 601 or the scanline driver circuit portion 603, various types of circuits such as aCMOS circuit, a PMOS circuit, or an NMOS circuit can be used. Although adriver in which a driver circuit portion is formed and a pixel areformed over the same surface of a substrate in the display device ofthis embodiment, the driver circuit portion is not necessarily formedover the substrate and can be formed outside the substrate.

The pixel portion 602 includes a switching transistor 611, a currentcontrol transistor 612, and a lower electrode 613 electrically connectedto a drain of the current control transistor 612. Note that a partitionwall 614 is formed to cover end portions of the lower electrode 613. Asthe partition wall 614, for example, a positive type photosensitiveacrylic resin film can be used.

In order to obtain favorable coverage by a film which is formed over thepartition wall 614, the partition wall 614 is formed to have a curvedsurface with curvature at its upper or lower end portion. For example,in the case of using a positive photosensitive acrylic as a material ofthe partition wall 614, it is preferable that only the upper end portionof the partition wall 614 have a curved surface with curvature (theradius of the curvature being 0.2 μm to 3 μm). As the partition wall614, either a negative photosensitive resin or a positive photosensitiveresin can be used.

Note that there is no particular limitation on a structure of each ofthe transistors (the transistors 611, 612, 623, and 624). For example, astaggered transistor can be used. In addition, there is no particularlimitation on the polarity of these transistors. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for these transistors. For example, an amorphoussemiconductor film or a crystalline semiconductor film may be used.Examples of a semiconductor material include Group 14 semiconductors(e.g., a semiconductor including silicon), compound semiconductors(including oxide semiconductors), organic semiconductors, and the like.For example, it is preferable to use an oxide semiconductor that has anenergy gap of 2 eV or more, preferably 2.5 eV or more and furtherpreferably 3 eV or more, for the transistors, so that the off-statecurrent of the transistors can be reduced. Examples of the oxidesemiconductor include an In—Ga oxide and an In-M-Zn oxide (M is aluminum(Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium(Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)).

An EL layer 616 and an upper electrode 617 are formed over the lowerelectrode 613. Here, the lower electrode 613 functions as an anode andthe upper electrode 617 functions as a cathode.

In addition, the EL layer 616 is formed by various methods such as anevaporation method with an evaporation mask, an ink-jet method, or aspin coating method. As another material included in the EL layer 616, alow molecular compound or a high molecular compound (including anoligomer or a dendrimer) may be used.

Note that a light-emitting element 618 is formed with the lowerelectrode 613, the EL layer 616, and the upper electrode 617. Thelight-emitting element 618 has any of the structures described inEmbodiments 1 to 3. In the case where the pixel portion includes aplurality of light-emitting elements, the pixel portion may include bothany of the light-emitting elements described in Embodiments 1 to 3 and alight-emitting element having a different structure.

When the sealing substrate 604 and the element substrate 610 areattached to each other with the sealant 605, the light-emitting element618 is provided in the region 607 surrounded by the element substrate610, the sealing substrate 604, and the sealant 605. The region 607 isfilled with a filler. In some cases, the region 607 is filled with aninert gas (nitrogen, argon, or the like) or filled with an ultravioletcurable resin or a thermosetting resin which can be used for the sealant605. For example, a polyvinyl chloride (PVC)-based resin, anacrylic-based resin, a polyimide-based resin, an epoxy-based resin, asilicone-based resin, a polyvinyl butyral (PVB)-based resin, or anethylene vinyl acetate (EVA)-based resin can be used. It is preferablethat the sealing substrate be provided with a recessed portion and thedesiccant be provided in the recessed portion, in which casedeterioration due to influence of moisture can be inhibited.

An optical element 621 is provided below the sealing substrate 604 tooverlap with the light-emitting element 618. A light-blocking layer 622is provided below the sealing substrate 604. The structures of theoptical element 621 and the light-blocking layer 622 can be the same asthose of the optical element and the light-blocking layer in Embodiment3, respectively.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is preferable that such a material do not transmit moisture oroxygen as much as possible. As the sealing substrate 604, a glasssubstrate, a quartz substrate, or a plastic substrate formed of fiberreinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester,acrylic, or the like can be used.

In the above-described manner, the display device including any of thelight-emitting elements and the optical elements which are described inEmbodiments 1 to 3 can be obtained

<Structure Example 2 of Display Device>

Next, another example of the display device is described with referenceto FIGS. 9A and 9B and FIG. 10. Note that FIGS. 9A and 9B and FIG. 10are each a cross-sectional view of a display device of one embodiment ofthe present invention.

In FIG. 9A, a substrate 1001, a base insulating film 1002, a gateinsulating film 1003, gate electrodes 1006, 1007, and 1008, a firstinterlayer insulating film 1020, a second interlayer insulating film1021, a peripheral portion 1042, a pixel portion 1040, a driver circuitportion 1041, lower electrodes 1024R, 1024G, and 1024B of light-emittingelements, a partition wall 1025, an EL layer 1028, an upper electrode1026 of the light-emitting elements, a sealing layer 1029, a sealingsubstrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 9A, examples of the optical elements, coloring layers (a redcoloring layer 1034R, a green coloring layer 1034G, and a blue coloringlayer 1034B) are provided on a transparent base material 1033. Further,a light-blocking layer 1035 may be provided. The transparent basematerial 1033 provided with the coloring layers and the light-blockinglayer is positioned and fixed to the substrate 1001. Note that thecoloring layers and the light-blocking layer are covered with anovercoat layer 1036. In the structure in FIG. 9A, red light, greenlight, and blue light transmit the coloring layers, and thus an imagecan be displayed with the use of pixels of three colors.

FIG. 9B illustrates an example in which, as examples of the opticalelements, the coloring layers (the red coloring layer 1034R, the greencoloring layer 1034G, and the blue coloring layer 1034B) are providedbetween the gate insulating film 1003 and the first interlayerinsulating film 1020. As in this structure, the coloring layers may beprovided between the substrate 1001 and the sealing substrate 1031.

FIG. 10 illustrates an example in which, as examples of the opticalelements, the coloring layers (the red coloring layer 1034R, the greencoloring layer 1034G, and the blue coloring layer 1034R) are providedbetween the first interlayer insulating film 1020 and the secondinterlayer insulating film 1021. As in this structure, the coloringlayers may be provided between the substrate 1001 and the sealingsubstrate 1031.

The above-described display device has a structure in which light isextracted from the substrate 1001 side where the transistors are formed(a bottom-emission structure), but may have a structure in which lightis extracted from the sealing substrate 1031 side (a top-emissionstructure).

<Structure Example 3 of Display Device>

FIGS. 11A and 11D are each an example of a cross-sectional view of adisplay device having a top emission structure. Note that FIGS. 11A and11B are each a cross-sectional view illustrating the display device ofone embodiment of the present invention, and the driver circuit portion1041, the peripheral portion 1042, and the like, which are illustratedin FIGS. 9A and 9B and FIG. 10, are not illustrated therein.

In this case, as the substrate 1001, a substrate that does not transmitlight can be used. The process up to the step of forming a connectionelectrode which connects the transistor and the anode of thelight-emitting element is performed in a manner similar to that of thedisplay device having a bottom-emission structure. Then, a thirdinterlayer insulating film 1037 is formed to cover an electrode 1022.This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed by using a materialsimilar to that of the second interlayer insulating film, or can beformed by using any other known materials.

The lower electrodes 1024R, 1024G, and 1024B of the light-emittingelements each function as an anode here, but may function as a cathode.Further, in the case of a display device having a top-emission structureas illustrated in FIGS. 11A and 11B, the lower electrodes 1024R, 1024G,and 1024B preferably have a function of reflecting light. The upperelectrode 1026 is provided over the EL layer 1028. It is preferable thatthe upper electrode 1026 have a function of reflecting light and afunction of transmitting light and that a microcavity structure be usedbetween the upper electrode 1026 and the lower electrodes 1024R, 1024G,and 1024B, in which case the intensity of light having a specificwavelength is increased.

In the case of a top-emission structure as illustrated in FIG. 11A,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the light-blocking layer 1035 whichis positioned between pixels. Note that a light-transmitting substrateis favorably used as the sealing substrate 1031.

FIG. 11A illustrates the structure provided with the light-emittingelements and the coloring layers for the light-emitting elements as anexample; however, the structure is not limited thereto. For example, asshown in FIG. 11B, a structure including the red coloring layer 1034Rand the blue coloring layer 1034B but not including a green coloringlayer may be employed to achieve full color display with the threecolors of red, green, and blue. The structure as illustrated in FIG. 11Awhere the light-emitting elements are provided with the coloring layersis effective to suppress reflection of external light. In contrast, thestructure as illustrated in FIG. 11B where the light-emitting elementsare provided with the red coloring layer and the blue coloring layer andwithout the green coloring layer is effective to reduce powerconsumption because of small energy loss of light emitted from the greenlight-emitting element.

<Structure Example 4 of Display Device>

Although a display device including sub-pixels of three colors (red,green, and blue) is described above, the number of colors of sub-pixelsmay be four (red, green, blue, and yellow, or red, green, blue, andwhite). FIGS. 12A and 12B, FIG. 13, and FIGS. 14A and 14B illustratestructures of display devices each including the lower electrodes 1024R,1024G, 1024B, and 1024Y. FIGS. 12A and 12B and FIG. 13 each illustrate adisplay device having a structure in which light is extracted from thesubstrata 1001 side on which transistors are formed (bottom-emissionstructure), and FIGS. 14A and 14B each illustrate a display devicehaving a structure in which light is extracted from the sealingsubstrate 1031 side (top-emission structure).

FIG. 12A illustrates an example of a display device in which opticalelements (the coloring layer 1034R, the coloring layer 1034G, thecoloring layer 1034B, and a coloring layer 1034Y) are provided on thetransparent base material 1033. FIG. 12B illustrates an example of adisplay device in which optical elements (the coloring layer 1034R, thecoloring layer 1034G, and the coloring layer 1034B) are provided betweenthe gate insulating film 1003 and the first interlayer insulating film1020. FIG. 13 illustrates an example of a display device in whichoptical elements (the coloring layer 1034R, the coloring layer 1034G,the coloring layer 1034B, and the coloring layer 1034Y) are providedbetween the first interlayer insulating film 1020 and the secondinterlayer insulating film 1021.

The coloring layer 1034R transmits red light, the coloring layer 1034Gtransmits green light, and the coloring layer 1034B transmits bluelight. The coloring layer 1034Y transmits yellow light or transmitslight of a plurality of colors selected from blue, green, yellow, andred. When the coloring layer 1034Y can transmit light of a plurality ofcolors selected from blue, green, yellow, and red, light released fromthe coloring layer 1034Y may be white light. Since the light-emittingelement which transmits yellow or white light has high emissionefficiency, the display device including the coloring layer 1034Y canhave lower power consumption.

In the top-emission display devices illustrated in FIGS. 14A and 14B, alight-emitting element including the lower electrode 1024Y preferablyhas a microcavity structure between the upper electrode 1026 and thelower electrodes 1024R, 1024G, 1024B, and 1024Y as in the display deviceillustrated in FIG. 11A. In the display device illustrated in FIG. 14A,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, the blue coloring layer 1034B, and the yellow coloring layer1034Y) are provided.

Light emitted through the microcavity and the yellow coloring layer1034Y has an emission spectrum in a yellow region. Since yellow is acolor with a high luminosity factor, a light-emitting element emittingyellow light has high emission efficiency. Therefore, the display deviceof FIG. 14A can reduce power consumption.

FIG. 14A illustrates the structure provided with the light-emittingelements and the coloring layers for the light-emitting elements as anexample; however, the structure is not limited thereto. For example, asshown in FIG. 14B, a structure including the red coloring layer 1034R,the green coloring layer 1034G, and the blue coloring layer 1034B butnot including a yellow coloring layer may be employed to achieve fullcolor display with the four colors of red, green, blue, and yellow or ofred, green, blue, and white. The structure as illustrated in FIG. 14Awhere the light-emitting elements are provided with the coloring layersis effective to suppress reflection of external light. In contrast, thestructure as illustrated in FIG. 14B where the light-emitting elementsare provided with the red coloring layer, the green coloring layer, andthe blue coloring layer and without the yellow coloring layer iseffective to reduce power consumption because of small energy loss oflight emitted from the yellow or white light-emitting element.

The structure described in this embodiment can be combined with any ofthe structures in this embodiment and the other embodiments.

Embodiment 5

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention will be described withreference to FIGS. 15A and 15B, FIGS. 16A and 16B, and FIGS. 17A and17B.

FIG. 15A is a block diagram illustrating the display device of oneembodiment of the present invention, and FIG. 15B is a circuit diagramillustrating a pixel circuit of the display device of one embodiment ofthe present invention.

<Description of Display Device>

The display device illustrated in FIG. 15A includes a region includingpixels of display elements (the region is hereinafter referred to as apixel portion 802), a circuit portion provided outside the pixel portion802 and including circuits for driving the pixels (the portion ishereinafter referred to as a driver circuit portion 804), circuitshaving a function of protecting elements (the circuits are hereinafterreferred to as protection circuits 806), and a terminal portion 807.Note that the protection circuits 806 are not necessarily provided.

A part or the whole of the driver circuit portion 804 is preferablyformed over a substrate over which the pixel portion 802 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 804 isnot formed over the substrate over which the pixel portion 802 isformed, the part or the whole of the driver circuit portion 804 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 802 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (such circuits arehereinafter referred to as pixel circuits 801). The driver circuitportion 804 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (the circuit is hereinafterreferred to as a scan line driver circuit 804 a) and a circuit forsupplying a signal (data signal) to drive a display element in a pixel(the circuit is hereinafter referred to as a signal line driver circuit804 b).

The scan line driver circuit 804 a includes a shift register or thelike. Through the terminal portion 807, the scan line driver circuit 804a receives a signal for driving the shift register and outputs a signal.For example, the scan line driver circuit 804 a receives a start pulsesignal, a clock signal, or the like and outputs a pulse signal. The scanline driver circuit 804 a has a function of controlling the potentialsof wirings supplied with scan signals (such wirings are hereinafterreferred to as scan lines GL_1 to GL_X). Note that a plurality of scanline driver circuits 804 a may be provided to control the scan linesGL_1 to GL_X separately. Alternatively, the scan line driver circuit 804a has a function of supplying an initialization signal. Without beinglimited thereto, the scan line driver circuit 804 a can supply anothersignal.

The signal line driver circuit 804 b includes a shift register or thelike. The signal line driver circuit 804 b receives a signal (imagesignal) from which a data signal is derived, as well as a signal fordriving the shift register, through the terminal portion 807. The signalline driver circuit 804 b has a function of generating a data signal tobe written to the pixel circuit 801 which is based on the image signal.In addition, the signal line driver circuit 804 b has a function ofcontrolling output of a data signal in response to a pulse signalproduced by input of a start pulse signal, a clock signal, or the like.Furthermore, the signal line driver circuit 804 b has a function ofcontrolling the potentials of wirings supplied with data signals (suchwirings are hereinafter referred to as data lines DL_1 to DL_Y).Alternatively, the signal line driver circuit 804 b has a function ofsupplying an initialization signal. Without being limited thereto, thesignal line driver circuit 804 b can supply another signal.

The signal line driver circuit 804 b includes a plurality of analogswitches or the like, for example. The signal line driver circuit 804 bcan output, as the data signals, signals obtained by time-dividing theimage signal by sequentially turning on the plurality of analogswitches. The signal line driver circuit 804 b may include a shiftregister or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 801 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 801 arecontrolled by the scan line driver circuit 804 a. For example, to thepixel circuit 801 in the m-th row and the n-th column (m is a naturalnumber of less than or equal to X, and n is a natural number of lessthan or equal to Y), a pulse signal is input from the scan line drivercircuit 804 a through the scan line GL_m, and a data signal is inputfrom the signal line driver circuit 804 b through the data line DL n inaccordance with the potential of the scan line GL_m.

The protection circuit 806 shown in FIG. 15A is connected to, forexample, the scan line GL between the scan line driver circuit 804 a andthe pixel circuit 801. Alternatively, the protection circuit 806 isconnected to the data line DL between the signal line driver circuit 804b and the pixel circuit 801. Alternatively, the protection circuit 806can be connected to a wiring between the scan line driver circuit 804 aand the terminal portion 807. Alternatively, the protection circuit 806can be connected to a wiring between the signal line driver circuit 804b and the terminal portion 807. Note that the terminal portion 807 meansa portion having terminals for inputting power, control signals, andimage signals to the display device from external circuits.

The protection circuit 806 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 15A, the protection circuits 806 are provided forthe pixel portion 802 and the driver circuit portion 804, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 806 is not limited to that, andfor example, a configuration in which the protection circuits 806 areconnected to the scan line driver circuit 804 a or a configuration inwhich the protection circuits 806 are connected to the signal linedriver circuit 804 b may be employed. Alternatively, the protectioncircuits 806 may be configured to be connected to the terminal portion807.

In FIG. 15A, an example in which the driver circuit portion 804 includesthe scan line driver circuit 804 a and the signal line driver circuit804 b is shown; however, the structure is not limited thereto. Forexample, only the scan line driver circuit 804 a may be formed and aseparately prepared substrate where a signal line driver circuit isformed (e.g., a driver circuit substrate formed with a single crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted.

<Structural Example of Pixel Circuit>

Each of the plurality of pixel circuits 801 in FIG. 15A can have astructure illustrated in FIG. 15B, for example.

The pixel circuit 801 illustrated in FIG. 15B includes transistors 852and 854, a capacitor 862, and a light-emitting element 872.

One of a source electrode and a drain electrode of the transistor 852 iselectrically connected to a wiring to which a data signal is supplied (adata line DL_n). A gate electrode of the transistor 852 is electricallyconnected to a wiring to which a gate signal is supplied (a scan lineGL_m).

The transistor 852 has a function of controlling whether to write a datasignal.

One of a pair of electrodes of the capacitor 862 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 852.

The capacitor 862 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 854 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 854 is electrically connected to theother of the source electrode and the drain electrode of the transistor852.

One of an anode and a cathode of the light-emitting element 872 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 854.

As the light-emitting element 872, any of the light-emitting elementsdescribed in Embodiments 1 to 3 can be used.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel circuits 801 in FIG. 15B, thepixel circuits 801 are sequentially selected row by row by the scan linedriver circuit 804 a in FIG. 15A, for example, whereby the transistors852 are turned on and a data signal is written.

When the transistors 852 are turned off, the pixel circuits 801 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 854 is controlled in accordance with thepotential of the written data signal. The light-emitting element 872emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

Alternatively, the pixel circuit can have a function of compensatingvariation in threshold voltages or the like of a transistor. FIGS. 16Aand 163 and FIGS. 17A and 17B illustrate examples of the pixel circuit.

The pixel circuit illustrated in FIG. 16A includes six transistors(transistors 303_1 to 303_6), a capacitor 304, and a light-emittingelement 305. The pixel circuit illustrated in FIG. 16A is electricallyconnected to wirings 301_1 to 301_5 and wirings 302_1 and 302_2. Notethat as the transistors 303_1 to 3036, for example, p-channeltransistors can be used.

The pixel circuit shown in FIG. 16B has a configuration in which atransistor 303_7 is added to the pixel circuit shown in FIG. 16A. Thepixel circuit illustrated in FIG. 16B is electrically connected towirings 301_6 and 301_7. The wirings 301_5 and 301_6 may be electricallyconnected to each other. Note that as the transistor 303 7, for example,a p-channel transistor can be used.

The pixel circuit shown in FIG. 17A includes six transistors(transistors 308_1 to 308 6), the capacitor 304, and the light-emittingelement 305. The pixel circuit illustrated in FIG. 17A is electricallyconnected to wirings 306_1 to 306_3 and wirings 307_1 to 307_3. Thewirings 306_1 and 306_3 may be electrically connected to each other.Note that as the transistors 308_1 to 308_6, for example, p-channeltransistors can be used.

The pixel circuit illustrated in FIG. 17B includes two transistors(transistors 309_1 and 309_2), two capacitors (capacitors 304_1 and304_2), and the light-emitting element 305. The pixel circuitillustrated in FIG. 17B is electrically connected to wirings 311_1 to311_3 and wirings 312_1 and 312_2. With the configuration of the pixelcircuit illustrated in FIG. 17B, the pixel circuit can be driven by avoltage inputting current driving method (also referred to as CVCC).Note that as the transistors 309_1 and 309_2, for example, p-channeltransistors can be used.

A light-emitting element of one embodiment of the present invention canbe used for an active matrix method in which an active element isincluded in a pixel of a display device or a passive matrix method inwhich an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also a variety of active elements(non-linear elements) can be used. For example, a metal insulator metal(MIM), a thin film diode (TFD), or the like can also be used. Sincethese elements can be formed with a smaller number of manufacturingsteps, manufacturing cost can be reduced to yield can be improved.Alternatively, since the size of these elements is small, the apertureratio can be improved, so that power consumption can be reduced andhigher luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or yield can be improved. Alternatively, since anactive element (a non-linear element) is not used, the aperture ratiocan be improved, so that power consumption can be reduced or higherluminance can be achieved, for example.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention and an electronic device inwhich the display device is provided with an input device will bedescribed with reference to FIGS. 18A and 18B, FIGS. 19A to 19C, FIGS.20A and 20B, FIGS. 21A and 21B, and FIG. 22.

<Description 1 of Touch Panel>

In this embodiment, a touch panel 2000 including a display device and aninput device will be described as an example of an electronic device. Inaddition, an example in which a touch sensor is used as an input devicewill be described.

FIGS. 18A and 18B are perspective views of the touch panel 2000. Notethat FIGS. 18A and 18B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor2595 (see FIG. 18B). The touch panel 2000 also includes a substrata2510, a substrata 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality of wirings 2511 through which signals aresupplied to the pixels. The plurality of wirings 2511 are led to aperipheral portion of the substrate 2510, and parts of the plurality ofwirings 2511 form a terminal 2519. The terminal 2519 is electricallyconnected to an FPC 2509(1). The plurality of wirings 2511 can supplysignals from a signal line driver circuit 2503 s(1) to the plurality ofpixels.

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 18B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used.Examples of the capacitive touch sensor are a surface capacitive touchsensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. The use of a mutual capacitive type is preferablebecause multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 18B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense approach or contact of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 18A and 18B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

<Description of Display Device>

Next, the display device 2501 will be described in detail with referenceto FIG. 19A. FIG. 19A corresponds to a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 18B.

The display device 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

In the following description, an example of using a light-emittingelement that emits white light as a display element will be described;however, the display element is not limited to such an element. Forexample, light-emitting elements that emit light of different colors maybe included so that the light of different colors can be emitted fromadjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability of lower than or equal to 1×10⁻⁵g·m⁻²·day⁻¹, preferably lower than or equal to 1×10⁻⁶ g·m⁻²·day⁻¹ can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate 2510 and the substrate 2570. For example, thecoefficients of linear expansion of the materials are preferably lowerthan or equal to 1×10⁻³/K, further preferably lower than or equal to5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the light-emittingelement, a flexible substrate 2510 b, and an adhesive layer 2510 c forattaching the insulating layer 2510 a and the flexible substrate 2510 bto each other. The substrate 2570 is a stacked body including aninsulating layer 2570 a for preventing impurity diffusion into thelight-emitting element, a flexible substrate 2570 b, and an adhesivelayer 2570 c for attaching the insulating layer 2570 a and the flexiblesubstrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, or acrylic, urethane, or epoxy can be used.Alternatively, a material that includes a resin having a siloxane bondcan be used.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 19A, the sealing layer2560 can also serve as an optical adhesive layer.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, a light-emitting element 2550R can beprovided in a region surrounded by the substrate 2510, the substrate2570, the sealing layer 2560, and the sealant. Note that an inert gas(such as nitrogen and argon) may be used instead of the sealing layer2560. A drying agent may be provided in the inert gas so as to adsorbmoisture or the like. An epoxy-based resin or a glass frit is preferablyused as the sealant. As a material used for the sealant, a materialwhich is impermeable to moisture and oxygen is preferably used.

The display device 2501 includes a pixel 2502R. The pixel 2502R includesa light-emitting module 2580R.

The pixel 2502R includes the light-emitting element 2550R and atransistor 25021 that can supply electric power to the light-emittingelement 2550R. Note that the transistor 2502 t functions as part of thepixel circuit. The light-emitting module 2580R includes thelight-emitting element 2550R and a coloring layer 2567R.

The light-emitting element 2550R includes a lower electrode, an upperelectrode, and an EL layer between the lower electrode and the upperelectrode. As the light-emitting element 2550R, any of thelight-emitting elements described in Embodiments 1 to 3 can be used.

A microcavity structure may be employed between the lower electrode andthe upper electrode so as to increase the intensity of light having aspecific wavelength.

In the case where the sealing layer 2560 is provided on the lightextraction side, the sealing layer 2560 is in contact with thelight-emitting element 2550R and the coloring layer 2567R.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. Accordingly, part of light emitted fromthe light-emitting element 2550R passes through the coloring layer 2567Rand is emitted to the outside of the light-emitting module 2580R asindicated by an arrow in FIG. 19A.

The display device 2501 includes a light-blocking layer 2567BM on thelight extraction side. The light-blocking layer 2567BM is provided so asto surround the coloring layer 2567R.

The coloring layer 2567R is a coloring layer having a function oftransmitting light in a particular wavelength range. For example, acolor filter for transmitting light in a red wavelength range, a colorfilter for transmitting light in a green wavelength range, a colorfilter for transmitting light in a blue wavelength range, a color filterfor transmitting light in a yellow wavelength range, or the like can beused. Each color filter can be formed with any of various materials by aprinting method, an inkjet method, an etching method using aphotolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. Theinsulating layer 2521 covers the transistor 2502 t. Note that theinsulating layer 2521 has a function of covering unevenness caused bythe pixel circuit. The insulating layer 2521 may have a function ofsuppressing impurity diffusion. This can prevent the reliability of thetransistor 2502 t or the like from being lowered by impurity diffusion.

The light-emitting element 2550R is formed over the insulating layer2521. A partition 2528 is provided so as to overlap with an end portionof the lower electrode of the light-emitting element 2550R. Note that aspacer for controlling the distance between the substrate 2510 and thesubstrate 2570 may be formed over the partition 2528.

A scan line driver circuit 2503 g(1) includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the sameprocess and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a PWB.

In the display device 2501, transistors with any of a variety ofstructures can be used. FIG. 19A illustrates an example of usingbottom-gate transistors; however, the present invention is not limitedto this example, and top-gate transistors may be used in the displaydevice 2501 as illustrated in FIG. 19B.

In addition, there is no particular limitation on the polarity of thetransistor 2502 t and the transistor 2503 t. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for the transistors 2502 t and 2503 t. Forexample, an amorphous semiconductor film or a crystalline semiconductorfilm may be used. Examples of semiconductor materials include Group 14semiconductors (e.g., a semiconductor including silicon), compoundsemiconductors (including oxide semiconductors), organic semiconductors,and the like. An oxide semiconductor that has an energy gap of 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more ispreferably used for one of the transistors 2502 t and 2503 t or both, sothat the off-state currant of the transistors can be reduced. Examplesof the oxide semiconductors include an In—Ga oxide, an In—M—Zn oxide (Mrepresents Al, Ga, Y, Zr, La, Ce, Sn, Hf, or Nd), and the like.

<Description of Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 19C. FIG. 19C corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 18B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes 2591 and the electrodes2592, and the wiring 2594 that electrically connects the adjacentelectrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film including graphene may be used aswell. The film including graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various pattern forming techniques such asphotolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond, andan inorganic insulating material such as silicon oxide, siliconoxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with higher conductivity than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), and the like can beused.

<Description 2 of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 20A. FIG. 20A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 18A.

In the touch panel 2000 illustrated in FIG. 20A, the display device 2501described with reference to FIG. 19A and the touch sensor 2595 describedwith reference to FIG. 19C are attached to each other.

The touch panel 2000 illustrated in FIG. 20A includes an adhesive layer2597 and an anti-reflective layer 2567 p in addition to the componentsdescribed with reference to FIGS. 19A and 19C.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaydevice 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2567 p is positioned in a region overlappingwith pixels. As the anti-reflective layer 2567 p, a circularlypolarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 20A will be described with reference to FIG. 20B.

FIG. 20B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 20B differs from the touch panel 2000illustrated in FIG. 20A in the position of the touch sensor 2595relative to the display device 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. The light-emitting element 2550Rillustrated in FIG. 20B emits light to the side where the transistor2502 t is provided. Accordingly, part of light emitted from thelight-emitting element 2550R passes through the coloring layer 2567R andis emitted to the outside of the light-emitting module 2580R asindicated by an arrow in FIG. 20B.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display device2501.

As illustrated in FIG. 20A or 20B, light may be emitted from thelight-emitting element to one or both of upper and lower sides of thesubstrate 2510.

<Description of Method for Driving Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 21A and 21B.

FIG. 21A is a block diagram illustrating the structure of a mutualcapacitive touch sensor FIG. 21A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 21A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current FIG. 21A also illustrates capacitors2603 that are each formed in a region where the electrodes 2621 and 2622overlap with each other. Note that functional replacement between theelectrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 21B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 21A. In FIG. 21B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 21B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). In FIG. 21B, sensed current values of the wirings Y1 to Y6are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change in accordance with changes inthe voltages of the wirings X1 to X6. The current value is decreased atthe point of approach or contact of a sensing target and accordingly thewaveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<Description of Sensor Circuit>

Although FIG. 21A illustrates a passive matrix type touch sensor inwhich only the capacitor 2603 is provided at the intersection of wiringsas a touch sensor, an active matrix type touch sensor including atransistor and a capacitor may be used. FIG. 22 illustrates an exampleof a sensor circuit included in an active matrix type touch sensor.

The sensor circuit in FIG. 22 includes the capacitor 2603 andtransistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 22 will be described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential with respect to the voltage VRES is thusapplied to the node n connected to the gate of the transistor 2611.Then, a potential for turning off the transistor 2613 is applied as thesignal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger, andaccordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductorlayer is preferably used as a semiconductor layer in which a channelregion is formed. In particular, such a transistor is preferably used asthe transistor 2613 so that the potential of the node n can be held fora long time and the frequency of operation of resupplying VRES to thenode n (refresh operation) can be reduced.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 7

In this embodiment, a display module and electronic devices including alight-emitting element of one embodiment of the present invention willbe described with reference to FIG. 23 and FIGS. 24A to 24G.

<Description of Display Module>

In a display module 8000 in FIG. 23, a touch sensor 8004 connected to anFPC 8003, a display device 8006 connected to an FPC 8005, a frame 8009,a printed board 8010, and a battery 8011 are provided between an uppercover 8001 and a lower cover 8002.

The light-emitting element of one embodiment of the present inventioncan be used for the display device 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchsensor 8004 and the display device 8006.

The touch sensor 8004 can be a resistive touch sensor or a capacitivetouch sensor and may be formed to overlap with the display device 8006.A counter substrate (sealing substrate) of the display device 8006 canhave a touch sensor function. A photosensor may be provided in eachpixel of the display device 8006 so that an optical touch sensor isobtained.

The frame 8009 protects the display device 8006 and also serves as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may serve as aradiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<Description of Electronic Device>

FIGS. 24A to 24G illustrate electronic devices. These electronic devicescan include a housing 9000, a display portion 9001, a speaker 9003,operation keys 9005 (including a power switch or an operation switch), aconnection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring or sensing force, displacement, position, speed, acceleration,angular velocity, rotational frequency, distance, light, liquid,magnetism, temperature, chemical substance, sound, time, hardness,electric field, current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared ray), a microphone9008, and the like.

The electronic devices illustrated in FIGS. 24A to 24G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch sensor function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions that can be provided for theelectronic devices illustrated in FIGS. 24A to 24G are not limited tothose described above, and the electronic devices can have a variety offunctions. Although not illustrated in FIGS. 24A to 24G, the electronicdevices may include a plurality of display portions. The electronicdevices may have a camera or the like and a function of taking a stillimage, a function of taking a moving image, a function of storing thetaken image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The electronic devices illustrated in FIGS. 24A to 24G will be describedin detail below.

FIG. 24A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible. Therefore, the display portion 9001 can be incorporated alonga bent surface of a bent housing 9000. In addition, the display portion9001 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, when anicon displayed on the display portion 9001 is touched, an applicationcan be started.

FIG. 24B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal can be used as asmartphone. Note that the speaker 9003, the connection terminal 9006,the sensor 9007, and the like, which are not shown in FIG. 24B, can bepositioned in the portable information terminal 9101 as in the portableinformation terminal 9100 shown in FIG. 24A. The portable informationterminal 9101 can display characters and image information on itsplurality of surfaces. For example, three operation buttons 9050 (alsoreferred to as operation icons, or simply, icons) can be displayed onone surface of the display portion 9001. Furthermore, information 9051indicated by dashed rectangles can be displayed on another surface ofthe display portion 9001. Examples of the information 9051 includedisplay indicating reception of an incoming email, social networkingservice (SNS) message, call, and the like; the title and sender of anemail and SNS message; the date; the time; remaining battery; and thereception strength of an antenna Instead of the information 9051, theoperation buttons 9050 or the like may be displayed on the positionwhere the information 9051 is displayed.

FIG. 24C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. Thus, the user can see the displaywithout taking out the portable information terminal 9102 from thepocket and decide whether to answer the call.

FIG. 24D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is bent, and images can be displayed on the bent displaysurface. The portable information terminal 9200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. The portable information terminal 9200 includes the connectionterminal 9006, and data can be directly transmitted to and received fromanother information terminal via a connector. Power charging through theconnection terminal 9006 is possible. Note that the charging operationmay be performed by wireless power feeding without using the connectionterminal 9006.

FIGS. 24E, 24F, and 24G are perspective views of a foldable portableinformation terminal 9201. FIG. 24E is a perspective view illustratingthe portable information terminal 9201 that is opened. FIG. 24F is aperspective view illustrating the portable information terminal 9201that is being opened or being folded FIG. 24G is a perspective viewillustrating the portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from an opened state to a folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thelight-emitting element of one embodiment of the present invention canalso be used for an electronic device which does not have a displayportion. The structure in which the display portion of the electronicdevice described in this embodiment is flexible and display can beperformed on the bent display surface or the structure in which thedisplay portion of the electronic device is foldable is described as anexample; however, the structure is not limited thereto and a structurein which the display portion of the electronic device is not flexibleand display is performed on a plane portion may be employed.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 8

In this embodiment, a light-emitting device including the light-emittingelement of one embodiment of the present invention will be describedwith reference to FIGS. 25A to 25C and FIGS. 26A to 26D.

FIG. 25A is a perspective view of a light-emitting device 3000 shown inthis embodiment, and FIG. 25B is a cross-sectional view alongdashed-dotted line F-F in FIG. 25A. Note that in FIG. 25A, somecomponents are illustrated by broken lines in order to avoid complexityof the drawing.

The light-emitting device 3000 illustrated in FIGS. 25A and 25B includesa substrate 3001, a light-emitting element 3005 over the substrate 3001,a first sealing region 3007 provided around the light-emitting element3005, and a second sealing region 3009 provided around the first sealingregion 3007.

Light is emitted from the light-emitting element 3005 through one orboth of the substrate 3001 and a substrate 3003. In FIGS. 25A and 25B, astructure in which light is emitted from the light-emitting element 3005to the lower side (the substrate 3001 side) is illustrated.

As illustrated in FIGS. 25A and 25B, the light-emitting device 3000 hasa double sealing structure in which the light-emitting element 3005 issurrounded by the first sealing region 3007 and the second sealingregion 3009. With the double sealing structure, entry of impurities(e.g., water, oxygen, and the like) from the outside into thelight-emitting element 3005 can be favorably suppressed. Note that it isnot necessary to provide both the first sealing region 3007 and thesecond sealing region 3009. For example, only the first sealing region3007 may be provided.

Note that in FIG. 25B, the first sealing region 3007 and the secondsealing region 3009 are each provided in contact with the substrate 3001and the substrate 3003. However, without limitation to such a structure,for example, one or both of the first sealing region 3007 and the secondsealing region 3009 may be provided in contact with an insulating filmor a conductive film provided on the substrate 3001. Alternatively, oneor both of the first sealing region 3007 and the second sealing region3009 may be provided in contact with an insulating film or a conductivefilm provided on the substrate 3003.

The substrate 3001 and the substrate 3003 can have structures similar tothose of the substrate 200 and the substrate 220 described in Embodiment3, respectively. The light-emitting element 3005 can have a structuresimilar to that of any of the light-emitting elements described inEmbodiments 1 to 3.

For the first sealing region 3007, a material containing glass (e.g., aglass frit, a glass ribbon, and the like) can be used. For the secondsealing region 3009, a material containing a resin can be used. With theuse of the material containing glass for the first sealing region 3007,productivity and a sealing property can be improved. Moreover, with theuse of the material containing a resin for the second sealing region3009, impact resistance and heat resistance can be improved. However,the materials used for the first sealing region 3007 and the secondsealing region 3009 are not limited to such, and the first sealingregion 3007 may be formed using the material containing a resin and thesecond sealing region 3009 may be formed using the material containingglass.

The glass fit may contain, for example, magnesium oxide, calcium oxide,strontium oxide, barium oxide, cesium oxide, sodium oxide, potassiumoxide, boron oxide, vanadium oxide, zinc oxide, tellurium oxide,aluminum oxide, silicon dioxide, lead oxide, tin oxide, phosphorusoxide, ruthenium oxide, rhodium oxide, iron oxide, copper oxide,manganese dioxide, molybdenum oxide, niobium oxide, titanium oxide,tungsten oxide, bismuth oxide, zirconium oxide, lithium oxide, antimonyoxide, lead borate glass, tin phosphate glass, vanadate glass, orborosilicate glass. The glass frit preferably contains at least one kindof transition metal to absorb infrared light.

As the above glass frits, for example, a frit paste is applied to asubstrate and is subjected to heat treatment, laser light irradiation,or the like. The frit paste contains the glass frit and a resin (alsoreferred to as a binder) diluted by an organic solvent. Note that anabsorber which absorbs light having the wavelength of laser light may beadded to the glass frit. For example, an Nd:YAG laser or a semiconductorlaser is preferably used as the laser. The shape of laser light may becircular or quadrangular.

As the above material containing a resin, for example, materials thatinclude polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, an acrylic resin, urethane, an epoxy resin, ora resin having a siloxane bond can be used.

Note that in the case where the material containing glass is used forone or both of the first sealing region 3007 and the second sealingregion 3009, the material containing glass preferably has a thermalexpansion coefficient close to that of the substrate 3001. With theabove structure, generation of a crack in the material containing glassor the substrate 3001 due to thermal stress can be suppressed.

For example, the following advantageous effect can be obtained in thecase where the material containing glass is used for the first sealingregion 3007 and the material containing a resin is used for the secondsealing region 3009.

The second sealing region 3009 is provided closer to an outer portion ofthe light-emitting device 3000 than the first sealing region 3007 is. Inthe light-emitting device 3000, distortion due to external force or thelike increases toward the outer portion. Thus, the outer portion of thelight-emitting device 3000 where a larger amount of distortion isgenerated, that is, the second sealing region 3009 is sealed using thematerial containing a resin and the first sealing region 3007 providedon an inner side of the second sealing region 3009 is sealed using thematerial containing glass, whereby the light-emitting device 3000 isless likely to be damaged even when distortion due to external force orthe like is generated.

Furthermore, as illustrated in FIG. 25B, a first region 3011 correspondsto the region surrounded by the substrate 3001, the substrate 3003, thefirst sealing region 3007, and the second sealing region 3009. A secondregion 3013 corresponds to the region surrounded by the substrate 3001,the substrate 3003, the light-emitting element 3005, and the firstsealing region 3007.

The first region 3011 and the second region 3013 are preferably filledwith, for example, an inert gas such as a rare gas or a nitrogen gas.Note that for the first region 3011 and the second region 3013, areduced pressure state is preferred to an atmospheric pressure state.

FIG. 25C illustrates a modification example of the structure in FIG.25B. FIG. 25C is a cross-sectional view illustrating the modificationexample of the light-emitting device 3000.

FIG. 25C illustrates a structure in which a desiccant 3018 is providedin a recessed portion provided in part of the substrate 3003. The othercomponents are the same as those of the structure illustrated in FIG.25B.

As the desiccant 3018, a substance which adsorbs moisture and the likeby chemical adsorption or a substance which adsorbs moisture and thelike by physical adsorption can be used. Examples of the substance thatcan be used as the desiccant 3018 include alkali metal oxides, alkalineearth metal oxide (e.g., calcium oxide, barium oxide, and the like),sulfate, metal halides, perchlorate, zeolite, silica gel, and the like.

Next, modification examples of the light-emitting device 3000 which isillustrated in FIG. 25B are described with reference to FIGS. 26A to26D. Note that FIGS. 26A to 26D are cross-sectional views illustratingthe modification examples of the light-emitting device 3000 illustratedin FIG. 25B.

In each of the light-emitting devices illustrated in FIGS. 26A to 26D,the second sealing region 3009 is not provided but only the firstsealing region 3007 is provided. Moreover, in each of the light-emittingdevices illustrated in FIGS. 26A to 26D, a region 3014 is providedinstead of the second region 3013 illustrated in FIG. 25B.

For the region 3014, for example, materials that include polyester,polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate,an acrylic resin, an epoxy resin, urethane, an epoxy resin, or a resinhaving a siloxane bond can be used.

When the above-described material is used for the region 3014, what iscalled a solid-sealing light-emitting device can be obtained.

In the light-emitting device illustrated in FIG. 26B, a substrate 3015is provided on the substrate 3001 side of the light-emitting deviceillustrated in FIG. 26A.

The substrate 3015 has unevenness as illustrated in FIG. 26B. With astructure in which the substrate 3015 having unevenness is provided onthe side through which light emitted from the light-emitting element3005 is extracted, the efficiency of extraction of light from thelight-emitting element 3005 can be improved. Note that instead of thestructure having unevenness and illustrated in FIG. 26B, a substratehaving a function as a diffusion plate may be provided.

In the light-emitting device illustrated in FIG. 26C, light is extractedthrough the substrate 3003 side, unlike in the light-emitting deviceillustrated in FIG. 26A, in which light is extracted through thesubstrate 3001 side.

The light-emitting device illustrated in FIG. 26C includes the substrate3015 on the substrate 3003 side. The other components are the same asthose of the light-emitting device illustrated in FIG. 26B.

In the light-emitting device illustrated in FIG. 26D, the substrate 3003and the substrate 3015 included in the light-emitting device illustratedin FIG. 26C are not provided but a substrate 3016 is provided.

The substrate 3016 includes first unevenness positioned closer to thelight-emitting element 3005 and second unevenness positioned fartherfrom the light-emitting element 3005. With the structure illustrated inFIG. 26D, the efficiency of extraction of light from the light-emittingelement 3005 can be further improved.

Thus, the use of the structure described in this embodiment can providea light-emitting device in which deterioration of a light-emittingelement due to impurities such as moisture and oxygen is suppressed.Alternatively, with the structure described in this embodiment, alight-emitting device having high light extraction efficiency can beobtained.

Note that the structure described in this embodiment can be combinedwith the structure described in any of the other embodiments asappropriate.

Embodiment 9

In this embodiment, examples in which the light-emitting element of oneembodiment of the present invention is used for various lighting devicesand electronic devices will be described with reference to FIGS. 27A to27C and FIG. 28.

An electronic device or a lighting device that has a light-emittingregion with a curved surface can be obtained with the use of thelight-emitting element of one embodiment of the present invention whichis manufactured over a substrate having flexibility.

Furthermore, a light-emitting device to which one embodiment of thepresent invention is applied can also be used for lighting for motorvehicles, examples of which are lighting for a dashboard, a windshield,a ceiling, and the like.

FIG. 27A is a perspective view illustrating one surface of amultifunction terminal 3500, and FIG. 27B is a perspective viewillustrating the other surface of the multifunction terminal 3500. In ahousing 3502 of the multifunction terminal 3500, a display portion 3504,a camera 3506, lighting 3508, and the like are incorporated. Thelight-emitting device of one embodiment of the present invention can beused for the lighting 3508.

The lighting 3508 that includes the light-emitting device of oneembodiment of the present invention functions as a planar light source.Thus, unlike a point light source typified by an LED, the lighting 3508can provide light emission with low directivity. When the lighting 3508and the camera 3506 are used in combination, for example, imaging can beperformed by the camera 3506 with the lighting 3508 lighting orflashing. Because the lighting 3508 functions as a planar light source,a photograph as if taken under natural light can be taken.

Note that the multifunction terminal 3500 illustrated in FIGS. 27A and27B can have a variety of functions as in the electronic devicesillustrated in FIGS. 24A to 24G.

The housing 3502 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. When a detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor, isprovided inside the multifunction terminal 3500, display on the screenof the display portion 3504 can be automatically switched by determiningthe orientation of the multifunction terminal 3500 (whether themultifunction terminal is placed horizontally or vertically for alandscape mode or a portrait mode).

The display portion 3504 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 3504 is touched with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or a sensing light source which emits near-infrared light inthe display portion 3504, an image of a finger vein, a palm vein, or thelike can be taken. Note that the light-emitting device of one embodimentof the present invention may be used for the display portion 3504.

FIG. 27C is a perspective view of a security light 3600. The securitylight 3600 includes lighting 3608 on the outside of the housing 3602,and a speaker 3610 and the like are incorporated in the housing 3602.The light-emitting device of one embodiment of the present invention canbe used for the lighting 3608.

The security light 3600 emits light when the lighting 3608 is gripped orheld, for example. An electronic circuit that can control the manner oflight emission from the security light 3600 may be provided in thehousing 3602. The electronic circuit may be a circuit that enables lightemission once or intermittently plural times or may be a circuit thatcan adjust the amount of emitted light by controlling the current valuefor light emission. A circuit with which a loud audible alarm is outputfrom the speaker 3610 at the same time as light emission from thelighting 3608 may be incorporated.

The security light 3600 can emit light in various directions; therefore,it is possible to intimidate a thug or the like with light, or light andsound. Moreover, the security light 3600 may include a camera such as adigital still camera to have a photography function.

FIG. 28 illustrates an example in which the light-emitting element isused for an indoor lighting device 8501. Since the light-emittingelement can have a larger area, a lighting device having a large areacan also be formed. In addition, a lighting device 8502 in which alight-emitting region has a curved surface can also be formed with theuse of a housing with a curved surface. A light-emitting elementdescribed in this embodiment is in the form of a thin film, which allowsthe housing to be designed more freely. Therefore, the lighting devicecan be elaborately designed in a variety of ways. Furthermore, a wall ofthe room may be provided with a large-sized lighting device 8503. Touchsensors may be provided in the lighting devices 8501, 8502, and 8503 tocontrol the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side ofa table, a lighting device 8504 which has a function as a table can beobtained. When the light-emitting element is used as part of otherfurniture, a lighting device which has a function as the furniture canbe obtained.

As described above, lighting devices and electronic devices can beobtained by application of the light-emitting device of one embodimentof the present invention. Note that the light-emitting device can beused for lighting devices and electronic devices in a variety of fieldswithout being limited to the lighting devices and the electronic devicesdescribed in this embodiment.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Example

In this example, examples of fabricating light-emitting elements ofembodiments of the present invention will be described. Schematiccross-sectional views of light-emitting elements fabricated in thisexample are similar to that of the light-emitting element 250 in FIG.1A. Table 1 shows the detailed structures of the elements. In addition,structures and abbreviations of compounds used in this example are givenbelow.

TABLE 1 reference thickness layer numeral (nm) materials weight ratioLight-emitting electrode 102 200 Al — element 1 electron-injection layer119 1 LiF — electron-transport layer 118(2) 10 BPhen — 118(1) 204,6mCzP2Pm — light-emitting layer 130 40 4,6mCzP2Pm:PCCzTp:Rubrene0.8:0.2:0 01 hole-transport layer 112 20 BPAFLP — hole-injection layer111 20 DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO — Light-emittingelectrode 102 200 Al — element 2 electron-injection layer 119 1 LiF —electron-transport layer 118(2) 10 BPhen — 118(1) 20 4,6mCzP2Pm — lightemitting layer 130 40 4,6mCzP2Pm:FrBBiF II:Rubrene 0.8:0.2:0.01hole-transport layer 112 20 BPAFLP — hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO — Light-emitting electrode102 200 Al — element 3 electron-injection layer 119 1 LiF —electron-transport layer 118(2) 10 BPhen — 118(1) 20 4,6mCzP2Pm —light-emitting layer 130 40 4,6mCzP2Pm:PCBBiF:Rubrene 0.8:0.2:0.01hole-transport layer 112 20 BPAFLP — hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO — Light-emitting electrode102 200 Al — element 4 electron-injection layer 119 1 LiF —electron-transport layer 118(2) 10 BPhen — 118(1) 20 4,6mCzP2Pm —light-emitting layer 130 40 4,6mCzP2Pm:PCBiF:Rubrene 0.8:0.2:0.01hole-transport layer 112 20 BPAFLP — hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO —

<Fabrication of Light-Emitting Element 1>

As the electrode 101, an ITSO film was formed to a thickness of 110 nmover a substrate. The area of the electrode 101 was set to 4 mm² (2 mm 2mm).

Next, the EL layer 100 was formed over the electrode 101. As thehole-injection layer 111, 1,3,5-tri(dibenzothiophen-4-yl)benzene(abbreviation: DBT3P-II) and molybdenum oxide (MOO₃) were deposited byco-evaporation such that the deposited layer has a weight ratio ofDBT3P-II to MoO₃ of 1:0.5 and a thickness of nm. Then, as thehole-transport layer 112,4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)was deposited by evaporation to a thickness of 20 nm.

Next, as the light-emitting layer 130,4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mC7P2Pm), 9-phenyl-9′-(triphenylen-2-yl)-3,3′-bi-9H-carbazole(abbreviation: PCCzTp), and Rubrene were deposited by co-evaporationsuch that the deposited layer has a weight ratio of 4,6mCzP2Pm andPCCzTp to Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In thelight-emitting layer 130, 4,6mCzP2Pm and PCCzTp serve as the hostmaterial 131 and Rubrene serves as the guest material 132 (fluorescentmaterial).

As the elcctron-transport layer 118, 4,6mCzP2Pm and Bphen weresequentially deposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 130. Next, as theelectron-injection layer 119, lithium fluoride (LiF) was deposited byevaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was deposited to a thickness of 200nm.

Next, in a glove box containing a nitrogen atmosphere, Light-emittingelement 1 was sealed by fixing a sealing substrate to the substrateprovided with the EL layer 100 using a sealant for an organic EL device.Specifically, after the sealant was applied to surround the EL layer 100over the substrate and the substrate was bonded to the sealingsubstrate, irradiation with ultraviolet light having a wavelength of 365nm at 6 J/cm² and heat treatment at 80° C. for one hour were performed.Through the above process, Light-emitting element 1 was obtained.

<Fabrication of Light-Emitting Elements 2 to 4>

Light-emitting elements 2 to 4 are different from the above-describedLight-emitting element 1 in only the host material of the light-emittinglayer 130, and steps for the other components are the same as those in amethod for fabricating Light-emitting element 1.

As the light-emitting layer 130 of Light-emitting element 2, 4,6mCzP2Pm,N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II), and Rubrene were deposited by co-evaporationsuch that the deposited layer has a weight ratio of 4,6mCzP2Pm toFrBBiF-II and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In thelight-emitting layer 130, 4,6mCzP2Pm and FrBBiF-II serve as the hostmaterial 131 and Rubrene serves as the guest material 132 (fluorescentmaterial).

As the light-emitting layer 130 of Light-emitting element 3, 4,6mCzP2Pm,N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF), and Rubrene were deposited byco-evaporation such that the deposited layer has a weight ratio of4,6mCzP2Pm to PCBBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 40nm. In the light-emitting layer 130, 4,6mCzP2Pm and PCBBiF serve as thehost material 131 and Rubrene serves as the guest material 132(fluorescent material).

As the light-emitting layer 130 of Light-emitting element 4, 4,6mCzP2Pm,N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine(abbreviation: PCBiF), and Rubrene were deposited by co-evaporation suchthat the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBiF andRubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emittinglayer 130, 4,6mCzP2Pm and PCBiF serve as the host material 131 andRubrene serves as the guest material 132 (fluorescent material).

<Operation Characteristics 1 of Light-Emitting Elements>

Next, emission characteristics of the fabricated Light-emitting elements1 to 4 were measured. Note that the measurement was performed at roomtemperature (in an atmosphere kept at 23° C.).

The emission characteristics of the light-emitting elements at aluminance around 1000 cd/m² are shown below in Table 2. The currentefficiency-luminance characteristics, external quantumefficiency-luminance characteristics, and luminance-voltagecharacteristics of the light-emitting elements are shown in FIG. 29,FIG. 30, and FIG. 31, respectively. FIG. 32 shows electroluminescencespectra at the time when a current was made to flow in thelight-emitting elements at a current density of 2.5 mA/cm².

TABLE Current CIE Current Power External Voltage density chromaticityLuminance Efficiency Efficiency Quantrum (V) (mA/cm²) (x, y) (cd/m²)(cd/A) (lm/W) Efficiency (%) Light-emitting 4.4 6.1 (0.49, 0.49) 1000 1612 5.1 element 1 Light-emitting 4.0 5.3 (0.49, 0.50) 860 16 13 5.0element 2 Light-emitting 4.0 5.7 (0.48, 0.51) 1200 20 16 6.1 element 3Light-emitting 3.8 3.8 (0.48, 0.51) 1100 29 24 8.5 element 4

As shown by the peaks in the electroluminescence spectra in FIG. 32,only yellow light emission derived from Rubrene, which is a fluorescentmaterial, was observed from Light-emitting elements 1 to 4.

In addition, as shown in FIG. 29, FIG. 30, and Table 2, Light-emittingelements 1 to 4 have high current efficiency and high external quantumefficiency. In particular, Light-emitting element 4 has external quantumefficiency of higher than 8% at a luminance around 1000 cd/m².

Since the probability of formation of singlet excitons which aregenerated by recombination of carriers (holes and electrons) injectedfrom the pair of electrodes is at most 25%, the external quantumefficiency in the case where the light extraction efficiency to theoutside is 20% is at most 5%. Light-emitting elements 1 to 4 can haveexternal quantum efficiency of higher than 5%. This is becauseLight-emitting elements 1 to 4 emit, in addition to light originatingfrom singlet excitons generated by recombination of carriers (holes andelectrons) injected from the pair of electrodes, light originating fromsinglet excitons generated from triplet excitons by ExEF.

Moreover, as shown in FIG. 31 and Table 2, Light-emitting elements 1 to4 drive at a low voltage. That is, by including a light-emitting layerusing ExEF, a light-emitting element that drives at a low voltage can befabricated. Furthermore, a light-emitting element with reduced powerconsumption can be fabricated.

Next, to examine whether Light-emitting elements 1 to 4 emit light byExEF, light-emitting elements in which the guest material 132 is notcontained were fabricated and measured. Table 3 shows the detailedstructures of the elements.

TABLE 3 reference thickness layer numeral (nm) materials weight ratioLight-emitting electrode 102 200 Al — element 5 electron-injection layer119 1 LiF — electron-transport layer 118(2) 10 BPhen — 118(1) 204,6mCzP2Pm — light-emitting layer 130 40 4,6mCzP2Pm:PCCzTp 0.8:0.2hole-transport layer 112 20 BPAFLP — hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO — Light-emitting electrode102 200 Al — element 6 electron-injection layer 119 1 LiF —electron-transport layer 118(2) 10 BPhen — 118(1) 20 4,6mCzP2Pm —light-emitting layer 130 40 4,6mCzP2Pm:FrBBiF-II 0.8:0.2 hole-transportlayer 112 20 BPAFLP — hole-injection layer 111 20 DBT3P-II:MoO₃ 1:0.5electrode 101 110 ITSO — Light-emitting electrode 102 200 Al — element 7electron-injection layer 119 1 LiF — electron-transport layer 118(2) 10BPhen — 118(1) 20 4,6mCzP2Pm — light-emitting layer 130 404,6mCzP2Pm:PCBBiF 0.8:0.2 hole-transport layer 112 20 BPAFLP —hole-injection layer 111 20 DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO —Light-emitting electrode 102 200 Al — element 8 electron-injection layer119 1 LiF — electron-transport layer 118(2) 10 BPhen — 118(1) 204,6mCzP2Pm — light-emitting layer 130 40 4,6mCzP2Pm:PCBiF 0.8:0.2hole-transport layer 112 20 BPAFLP — hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 electrode 101 110 ITSO —

<Fabrication of Light-Emitting Elements 5 to 8>

Light-emitting elements 5 to 8 are different from the above-describedLight-emitting elements 1 to 4 in that the guest material 132 is notcontained in the light-emitting layer 130, and steps for the othercomponents are the same as those in methods for fabricatingLight-emitting elements 1 to 4.

As the light-emitting layer 130 of Light-emitting element 5, 4,6mCzP2Pmand PCCzTp were deposited by co-evaporation such that the depositedlayer has a weight ratio of 4,6mCzP2Pm to PCCzTp of 0.8:0.2 and athickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm andPCCzTp correspond to the host material 131 and the guest material 132 isnot contained.

As the light-emitting layer 130 of Light-emitting element 6, 4,6mCzP2Pmand FrBBiF-II were deposited by co-evaporation such that the depositedlayer has a weight ratio of 4,6mCzP2Pm to FrBBiF-II of 0.8:0.2 and athickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm andFrBBiF-II correspond to the host material 131 and the guest material 132is not contained.

As the light-emitting layer 130 of Light-emitting element 7, 4,6mCzP2Pmand PCBBiF were deposited by co-evaporation such that the depositedlayer has a weight ratio of 4,6mCzP2Pm to PCBBiF of 0.8:0.2 and athickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm andPCBBiF correspond to the host material 131 and the guest material 132 isnot contained.

As the light-emitting layer 130 of Light-emitting element 8, 4,6mCzP2Pmand PCBiF were deposited by co-evaporation such that the deposited layerhas a weight ratio of 4,6mCzP2Pm to PCBiF of 0.8:0.2 and a thickness of40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and PCBiF correspondto the host material 131 and the guest material 132 is not contained.

<Operation Characteristics 2 of Light-Emitting Elements>

Next, emission characteristics of the fabricated Light-emitting elements5 to 8 were measured. Note that the measurement was performed at roomtemperature (in an atmosphere kept at 23° C.).

The emission characteristics of the light-emitting elements at aluminance around 1000 cd/m² are shown below in Table 4. The currentefficiency-luminance characteristics, external quantumefficiency-luminance characteristics, and luminance-voltagecharacteristics of the light-emitting elements are shown in FIG. 33,FIG. 34, and FIG. 35, respectively. FIG. 36 shows electroluminescencespectra at the time when a current was made to flow in thelight-emitting elements at a current density of 2.5 mA/cm².

TABLE 4 Current CIE Current Power External Voltage density chromaticityLuminance Efficiency Efficiency Quantrum (V) (mA/cm²) (x, y) (cd/m²)(cd/A) (lm/W) Efficiency (%) Light-emitting 4.4 20 (0.22, 0.36) 950 4.83.4 2.0 element 5 Light-emitting 4.4 19 (0.26, 0.47) 970 5.1 3.7 1.7element 6 Light-emitting 3.7 5.2 (0.35, 0.56) 1000 20 17 6.1 element 7Light-emitting 3.5 2.9 (0.41, 0.56) 1100 38 34 11 element 8

FIG. 37 shows emission spectra of thin films of 4,6mCzP2Pm, PCCzTp,FrBBiF-II, PCBBiF, and PCBiF, which are used as host materials. Notethat the emission spectra of these thin films were measured with a PL-FLmeasurement apparatus (manufactured by Hamamatsu Photonics K.K.).

As shown in FIG. 37, from 4,6mCzP2Pm, PCCzTp, FrBBiF-II, PCBBiF, andPCBiF, which were used as host materials, blue light emissions havingpeak wavelengths of 493 nm, 418 nm, 428 nm, 436 nm, and 430 nm wereobserved, respectively.

In contrast, as shown in FIG. 36, from the electroluminescence spectrumpeaks of Light-emitting element 5, Light-emitting element 6,Light-emitting element 7, and Light-emitting element 8, green to yellowlight emissions having peak wavelengths of 499 nm, 513 nm, 536 nm, and552 nm were observed, respectively, and the full width at half maximumof any of the emission spectra is larger than those of the emissionspectra of individual compounds. The wavelengths of emission spectra ofLight-emitting element 5, Light-emitting element 6, Light-emittingelement 7, and Light-emitting element 8 correlate with the energydifferences between the LUMO level of 4,6mCzP2Pm and the HOMO level ofPCCzTp, between the LUMO level of 4,6mCzP2Pm and the HOMO level ofPrBBiF-II, between the LUMO level of 4,6mCzP2Pm and the HOMO level ofPCBBiF, and between the LUMO level of 4,6mCzP2Pm and the HOMO level ofPCBiF, respectively. Thus, Light-emitting element 5, Light-emittingelement 6. Light-emitting element 7, and Light-emitting element 8 emitlight emitted from exciplexes formed of 4,6mCzP2Pm and PCCzTp,4,6mCzP2Pm and FrBBiF-II, 4,6mCzP2Pm and PCBBiF, and 4,6mCzP2Pm andPCBiF, respectively.

In addition, FIG. 38 shows a measurement result of an absorptionspectrum of Rubrene in a toluene solution, which was used as a guestmaterial of Light-emitting elements 1 to 4.

As shown in FIG. 38, the absorption spectrum of Rubrene has anabsorption band which has a high molar absorption coefficient at around450 nm to 550 nm. This wavelength range substantially corresponds to thewavelength ranges of the electroluminescence spectra of the exciplexesshown in FIG. 36. Therefore, it is found that the compounds which forman exciplex is preferably used as the host material of Light-emittingelements 1 to 4 because the efficiency of energy transfer to the guestmaterial is increased.

In addition, as shown in FIG. 33, FIG. 34, and Table 4, Light-emittingelements 7 and 8 have high emission efficiency (current efficiency andexternal quantum efficiency). In particular, in Light-emitting element8, a drop (roll-off) in emission efficiency is small even on the highluminance side.

At a luminance around 1000 cd/m², the emission efficiency ofLight-emitting element 5 and the emission efficiency of Light-emittingelement 6 substantially correspond to each other, the emissionefficiency of Light-emitting element 7 is higher than those ofLight-emitting elements 5 and 6, and the emission efficiency ofLight-emitting element 8 is higher than that of Light-emitting element7. In addition, as for the above-described Light-emitting elements 1 to4, at a luminance around 1000 cd/m², the emission efficiency ofLight-emitting element 1 and the emission efficiency of Light-emittingelement 2 substantially correspond to each other, the emissionefficiency of Light-emitting element 3 is higher than those ofLight-emitting elements 1 and 2, and the emission efficiency ofLight-emitting element 4 is higher than that of Light-emitting element3.

That is, the emission efficiency of light emitted from the exciplexcorrelates with the emission efficiency of light emitted from the guestmaterial. The high emission efficiency of the exciplex means a smallrate constant of non-radiative deactivation in the exciplex, whichindicates a large rate constant of reverse intersystem crossing.

Then, the time-resolved fluorescence measurement was performed on thinfilms similar to light-emitting layers of Light-emitting elements 5 to8.

<Fabrication of Thin-Film Samples>

For the time-resolved fluorescence measurement of light-emitting layersof light-emitting elements, thin-film samples were fabricated on aquartz substrate by a vacuum evaporation method.

As Thin film 1, 4,6mCzP2Pm and FrBBiF-II were deposited byco-evaporation such that the deposited film has a weight ratio of4,6mCzP2Pm to FrBBiF-II of 0.8:0.2 and a thickness of 50 nm.

As Thin film 2, 4,6mCzP2Pm and PCBBiF were deposited by co-evaporationsuch that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBBiFof 0.8:0.2 and a thickness of 50 nm.

As Thin film 3, 4,6mCzP2Pm and PCBiF were deposited by co-evaporationsuch that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBiFof 0.8:0.2 and a thickness of 50 nm.

Thin films 1 to 3 were each sealed by fixing a sealing substrate to thequartz substrate over which the thin-film sample was deposited using asealant for an organic EL device in a glove box under a nitrogenatmosphere. Specifically, after a sealant was applied to surround eachof the thin films over the quartz substrate and the quartz substrate wasbonded to the sealing substrate, irradiation with ultraviolet lighthaving a wavelength of 365 nm at 6 J/cm² and heat treatment at 80° C.for one hour were performed.

<Time-Resolved Fluorescence Measurement of Thin-Film Samples>

A picosecond fluorescence lifetime measurement system (manufactured byHamamatsu Photonics K.K.) was used for the measurement. In thismeasurement, the thin film was irradiated with pulsed laser, andemission of the thin film which was attenuated from the laserirradiation underwent time-resolved measurement using a streak camera tomeasure the lifetime of fluorescent emission of the thin film. Anitrogen gas laser with a wavelength of 337 nm was used as the pulsedlaser. The thin film was irradiated with pulsed laser with a pulse widthof 500 ps at a repetition rate of 10 Hz. By integrating data obtained bythe repeated measurement, data with a high S/N ratio was obtained. Themeasurement was performed at room temperature (in an atmosphere kept at23° C.)

From each of Thin films 1 to 3, light emitted from an exciplex formed oftwo compounds was observed. The attenuation curves obtained by themeasurement are shown in FIGS. 39A and 39B.

The attenuation curves shown in FIGS. 39A and 39B were fitted withFormula (4).

$\begin{matrix}{L = {\sum\limits_{n = 1}{A_{n}{\exp\left( {- \frac{t}{a_{n}}} \right)}}}} & (4)\end{matrix}$

In Formula (4), L and t represent normalized emission intensity andelapsed time, respectively. The attenuation curve was able to be fittedwhen n was 1 and 2. This fitting results show that: the emissioncomponent of Thin film 1 contains a prompt fluorescent component havinga fluorescence lifetime of 0.60 μs and a delayed fluorescence componenthaving a fluorescence lifetime of 96 μs; the emission component of Thinfilm 2 contains a prompt fluorescent component having a fluorescencelifetime of 0.72 μs and a delayed fluorescence component having afluorescence lifetime of 55 μs; and the emission component of Thin film3 contains a prompt fluorescent component having a fluorescence lifetimeof 0.67 μs and a delayed fluorescence component having a fluorescencelifetime of 23 μs. That is, it is found that delayed fluorescencelifetimes of the exciplexes used in Light-emitting elements 4 and 8which have high efficiency are 50 μs or less, which is shorter thanthose of the exciplexes used in Light-emitting elements 2, 3, 6, and 7.In addition, the percentages of the delayed fluorescence component inlight emitted from Thin film 1, Thin film 2, and Thin film 3 werecalculated to 0.084%, 3.8%, and 9.4%, respectively.

As described above, Thin film 3 contains a delayed fluorescencecomponent having a relatively short fluorescence lifetime. The shortlifetime of the delayed fluorescence component indicates a large rateconstant of reverse intersystem crossing, which supports the resultsobtained from Light-emitting element 8. That is, triplet excitons in theexciplex are converted into singlet excitons in a relatively short time,whereby there is no saturation of exciton density even in ahigh-luminance region (a region where exciton density is high); thus,the emission efficiency of Light-emitting element 8 is less likely to bereduced in the high-luminance region. The comparison betweenLight-emitting elements 1 to 4 shows that the emission efficiency ofLight-emitting element 3 (the delayed fluorescence lifetime measuredusing the time-resolved fluorescence measurement of the exciplex is 55μs) is increased compared with Light-emitting element 1 and 2 and theemission efficiency of Light-emitting element 4 (the delayedfluorescence lifetime measured using the time-resolved fluorescencemeasurement of the exciplex is 23 μs) is significantly increased. Thus,in one embodiment of the present invention, the delayed fluorescencelifetime measured using the time-resolved fluorescence measurement ofthe exciplex is preferably 50 μs or less, further preferably 40 μs orless, still further preferably 30 μs or less.

In addition, the time-resolved fluorescence measurement shows that Thinfilm 3 contains the delayed fluorescence component at 5% or more oflight emitted from Thin film 3; thus, it indicates that the energytransfer between the singlet excited state and the triplet excited stateoccurs at a relatively high probability.

Furthermore, Light-emitting element 4 which contains an exciplexcontaining a short-life delayed fluorescence component as a hostmaterial has high emission efficiency. This is because the tripletexcitons in the exciplex are converted into the singlet excitons in arelatively short time, whereby the efficiency of energy transfer fromthe triplet excited state of the exciplex to the triplet excited stateof the guest material can be reduced and the generation efficiency ofthe singlet excitons in the exciplex and the guest material can beincreased.

As described above, compounds which form an exciplex are contained as ahost material and the exciplex contains a delayed fluorescence componenthaving a relatively short fluorescence lifetime of 50 μs or less at 5%or more, so that a light-emitting element having high emissionefficiency like Light-emitting element 4 can be fabricated.

The structure described in this example can be combined as appropriatewith any of the embodiments.

This application is based on Japanese Patent Application serial no.2015-046376 filed with Japan Patent Office on Mar. 9, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: alight-emitting layer comprising a first substance; a second substance;and a third substance, wherein the third substance is a fluorescentcompound, wherein emission of a mixture of the first substance and thesecond substance includes a delayed fluorescence component at roomtemperature, wherein a peak of the emission of the mixture comprises aregion overlapping with an absorption band on a lowest energy side ofthe third substance, wherein a highest occupied molecular orbital levelof the first substance is higher than a highest occupied molecularorbital level of the second substance, wherein a lowest unoccupiedmolecular orbital level of the first substance is higher than a lowestunoccupied molecular orbital level of the second substance, and whereina lifetime of the delayed fluorescence component is 10 ns or longer and50 ρs or shorter.
 2. The light-emitting element according to claim 1,wherein a fluorescence quantum yield of the third substance is 50% ormore.
 3. The light-emitting element according to claim 1, wherein ahighest occupied molecular orbital level of the first substance ishigher than a highest occupied molecular orbital level of the secondsubstance by 0.1 eV or more, and wherein a lowest unoccupied molecularorbital level of the first substance is higher than a lowest unoccupiedmolecular orbital level of the second substance by 0.1 eV or more, and4. The light-emitting element according to claim 1, wherein a weightratio of the third substance in the light-emitting layer is equal to orgreater than 0.001 and lower than or equal to 0.05.
 5. A light-emittingelement comprising: a light-emitting layer comprising a first substance;a second substance; and a third substance, wherein the third substanceis a fluorescent compound, wherein emission of a mixture of the firstsubstance and the second substance includes a delayed fluorescencecomponent at room temperature, wherein a peak of the delayedfluorescence component comprises a region overlapping with an absorptionband on a lowest energy side of the third substance, wherein a highestoccupied molecular orbital level of the first substance is higher than ahighest occupied molecular orbital level of the second substance,wherein a lowest unoccupied molecular orbital level of the firstsubstance is higher than a lowest unoccupied molecular orbital level ofthe second substance, and wherein a lifetime of the delayed fluorescencecomponent is 10 ns or longer and 50 μs or shorter.
 6. The light-emittingelement according to claim 5, wherein a fluorescence quantum yield ofthe third substance is 50% or more.
 7. The light-emitting elementaccording to claim 5, wherein a highest occupied molecular orbital levelof the first substance is higher than a highest occupied molecularorbital level of the second substance by 0.1 eV or more, and wherein alowest unoccupied molecular orbital level of the first substance ishigher than a lowest unoccupied molecular orbital level of the secondsubstance by 0.1 eV or more, and
 8. The light-emitting element accordingto claim 5, wherein a weight ratio of the third substance in thelight-emitting layer is equal to or greater than 0.001 and lower than orequal to 0.05.